To recline your seat a little is fine, but reclining it the WHOLE WAY, unless you are in business or first-class is totally rude. Yes, the seats were built to recline, but most of the planes used today are 20-25 years old and they used to have fewer seats then and much, much more leg room. (Ask anyone who's been flying for 25-30 years, like me, about the differences in air travel then.) The airline industry started trying to squeeze as many seats in as possible to make more money, at the passenger's travel comfort expense. On my last flight, the guy in front of me reclined the whole way for the entire flight making it impossible for me to use my tray table to work. If he had only reclined 1/2 of the way or even a 3/4 of the way, I would have had no problem. He never even bothered to look behind to see if I had any leg space at all - which I didn't - making my entire flight miserable.
???
They didn't add more (old style) seats to the planes they either made new planes or, when retrofitted, new seats. On the old planes (with more leg room), the seats reclined more than they did now. The amount of recline takes into account seat pitch (hence, BC or FC seats recline more).