I really don't see a problem with GF making exclusive content for USM that is excluded from Su/Mo. They do this all the time, it's not like they suddenly started doing this only with these games as pryce10 mentioned. If anything it's a pattern that we as pokemon fans should have memorized by now.
If this business tactic really bothers you guys this much, then remember it now and in the future always remember that when a new generation comes out there will be either a third game or a sequel game with more features. Don't buy the first game, save your money and wait till the third game or sequel comes out with the bonus features, that way by the end of it you'll have bought the game with all the bonus features without having to spend money on two games with the first one lacking in content.
Plus nobody is forcing anybody to buy USM. In fact to be honest, I was planning to skip USM. I actually promised myself that if USM didn't have amazing post game features, I was just going to skip it and wait for more news about Pokemon Switch (IMO, I think USM is already being overshadowed by Pokemon Switch, so I might just hold out for that.). I'm a patient dude with a tight bank account, so why pay for a game I've more or less already played?