The guidelines are "which ever comes first." So in your case, you'd need to do it at 6-months, not at 7,500 miles. Yes, if you do not follow these instructions, you do run the chance of voiding your warranty, but that really depends on the manufacturer and what they will allow. Some are a little easier than others, but they give you a maintanence schedule for a reason. Believe me when I say, if you don't follow it, it is a prime way for them to legally say, "Sorry, not our problem because you didn't complete the maintanence as scheduled." If they don't have to pay for something, they will find a way. And since you didn't do the maintanence correctly, they will use that against you.
As for a reason for actually changing the oil on time before the mileage comes about is that the additive packages in oil will break down over time, just as they will breakdown with mileage. You want to keep the level of protection your engine (or other mechanical parts) need. You can never go wrong with the routine maintanence.