I have looked into the ways that various types of tire pressure monitoring systems (TPMS) work. It seems that the typical factory installed active monitoring systems use pressure sensors that are mounted on the inside of the tire rims off the end of the valve stem assembly.
After market systems that I've seen so far have screw on caps for the valve stems that are the sensors. I deduce that for these to work they have to depress the post on the one way valve (a.k.a. Schrader Valve) to expose the cap sensor to the air pressure in the tire.
Are these after market type systems considered safe and are they legal in places such as the US? Since such system is so much easier to deploy than installing special valve stems with sensors inside the tire it would certainly be attractive to the car manufacturers, so are they driven by legal issues?