There's been some security-related questions being asked here and on sister sites concerning keyless ignition automobiles and how to defend against some security risks. This brings up a related question.
Why do cars keep running if the car can no longer detect the keyfob?
As JPhi1618 mentioned, this functionality centers around CFR 571.114, AKA FMVSS 114. Specifically section 5.2.1 which states:
Except as specified in S5.2.3, the starting system required by S5.1 must prevent key removal when tested according to the procedures in S6, unless the transmission or gear selection control is locked in “park” or becomes locked in “park” as a direct result of key removal.
An important point to understand is that car manufacturers consider the electronic code transmitted from the key-fob to the starting system the digital equivalent of a physical key, so the "removal" of the "key" occurs when the driver presses the button to shut off the car and the code is cleared from the starting system.
This would make it trivially easy to violate the regulation if the car cleared the start code as soon as the key fob was too far away, as it would allow a situation where the "key" can be "removed" before the car is put into park. So to skirt around that regulation, the car simply remains running so the car makers can argue that the "key" cannot be "removed" before the car is put into park. As an aside here, you could argue that it still violates the regulation, as many cars with push button start/stop will stop without the transmission being set to the "park" position when the start/stop button is pressed.
It looks like the last time CFR 571.114 was updated was in 2010, and it hasn't really kept up with this technology. There's a lot of grey area, lawsuits, and it'll probably be a few years before the law and technology fully mature here.
Edited to address @user1663987's comment: If the car engine stopped when the key-fob is moved too far away, then the car mfg would have to implement a system that physically prevents the driver from removing the key-fob from the car unless the system is in park to stay in compliance with the regulation. This would pretty much defeat the "key-less" point, as the best way to "lock" the key-fob in the car while it's running would be to just have a key on it and use a traditional key-based ignition. So in a sense, this behavior is a "desired" interpretation of the regs because it the car mfg doesn't have to kill a desirable selling point for their vehicle.
There is a safety aspect that can be considered, in the event that the key-fob is unexpectedly removed from the vehicle while driving (e.g.: child tosses it out the window), however this isn't something that's currently addressed in the regulations. It's possible that auto makers have looked into this already and found that a vehicle with a key-less ignition that continues running regardless of the proximity of the key-fob is safe, though reports of people leaving their vehicles running by mistake would indicate otherwise. At this point I've wandered into the realm of speculation, so I'll stop here.
I looked into this, and it turns out that almost all aspects of starting or turning off a car are governed by a federal requirements document called FMVSS 114. Things that are in there:
I can't find the most recent version of this document, but recent keyless ignition recalls and lawsuits have focused on this document.
Therefore, I say the reason this happens is because the government has mandated it to happen. I wish I could provide black and white proof. If anyone else finds a clear source post it in an answer or let me edit it into mine.
I found what looks like an older version of a compliance testing document, so that's something.
There may be other reasons but without a doubt a big part of it is because it's impossible to predict under what conditions the car no longer detects the key. And as stated in some of the comments, that could be while driving down the highway, or on the street at just the wrong moment.
Hope that helps!
It is pure and simple a safety thing.
The American law given above doesn't spell it out explicitly, but that is the goal of the given regulations - prevent accidental shutoffs while the vehicle is in motion so as to avoid potentially fatal accidents.
In Germany, the law is much the same though from what I've heard the phrasing is more explicit - the cars aren't allowed to shutoff unless the operator shuts it off in order to avoid accidents. That is to say, a car isn't allowed to automatically shut itself off.
The reason is quite simple:
The car doesn't know when it is safe to shutoff. As others have noted, it would be a bad thing to shutdown the motor on a highway.
The same reasoning applies to the keyless ignition system. If you shut off the car when the keyless fob goes out of range, then there's no telling where the vehicle is. So, the car must keep running until the operator shuts it off.
This is what makes the RSA attack on keyless systems possible. The thief uses a radio relay to make the car think the fob is close by, then starts the car and drives off. Once it is running, he can drive it anywhere as long as he doesn't stop the motor.
Anti-theft systems here also have to follow this law. If the thief can manage to start your car, then the law says he's got to be able to drive off with it and not have the motor shutoff while the car is being driven. This is not to help the thief, it is to protect other drivers when the thief gets out on the highway.
The anti-theft systems thing was bandied around here back in the 1990's. There were imported systems from the US that would shutoff the fuel pump if the car was hotwired - this is the same as having the motor shutoff. These imported anti-theft systems were illegal here.
I have heard that there were reports of cars rolling away due to drivers not turning the engine off with the car still in drive or reverse.Older ignition systems like in most cars on the roads today require the car to be placed in Park before removing the key.
Newer car systems with fobs are supposed to alert when the engine is still on.
