I got some interesting insight into this the other day. I picked up a Yukon from Hertz that had over 50,000 miles on it (only SUV they had). The agent had to get permission from Hertz' corporate office to rent it. While on hold, he explained that cars are automatically flagged by the system to be sold when they hit 50,000 miles. The computer won't allow them to re-rent it without some sort of approval.

Program vehicles (those that are bought back by the manufacturer) are usually removed from fleet with closer to 25,000 miles.

Oddly enough, according to paperwork in the glove box, this particular Yukon had a prior life as an Avis vehicle... transferred to Hertz with 24,000 miles on it already.