It's not so much "forcing" you as it is leveling the playing field. Look what would happen if one could get a FP for all three attractions that you want. At 60 days out, (plus length of ticket), those three attractions would book solid and "sell out". Every single person trying to get a FP inside of 59 days would get shut out and would have to wait in 3 long lines, while several thousand people wouldn't have to wait in any line at all. By tiering those attractions, almost every person in the park will be able to bypass one line and wait in two. Of course, under your proposed system, if you were lucky enough to be one of the few people who got to bypass all three lines, you'd love this. But if you missed out and were one of the many people who had to wait in all three lines, you'd be livid, and might even consider cancelling your trip. Disney doesn't want anyone to be so upset that they consider not going, and instead has created a system where nobody is "livid", and instead, everyone is a little bit frustrated. Universal frustration is a better system overall than one that creates a small subset of elated guests but a majority of peeved guests.
Yes, one ride. You can then use your other FPs on such E-ticket attractions as Figment, Spaceship Earth, and the Mickey/Minnie/Goofy meet-and-greet
