Ok, ive had this peeve for a while. My philosophy is that the reason I go to the gym is to do exercises that otherwise I cant (I dont have a bench at home, I dont have a power rack, I dont have 1000's of lbs to play around with). I can do pushups any time of the day, and at any location. Pullup bars are cheap and you can buy ones, or, you can go to a local park and find anything that will do.
I dont mind people doing chinups and stuff, but I find it weird that someone would waste their time in a gym to do pushups on the floor. Maybe people enjoy the atmosphere and find themselves being more motivated?
Its not my thing, but im not bothered by it so much as long as people arent in my way (when they never are) so its all good. To each his own.