You're right about the last part, which is why Fallout 2's southern map border ends at NCR. In response to the rest, I should point that Falloput's appeal is wandering about meeting interesting groups and resolvijg conflicts between them. Robots, mutans, cowboys, whacky 50s preconceptions... the trouble is that CL has NONE of these things and does not tolerate them. All traces of Pre-Legion identity, whether technological or cultural, are erased in favor of the Legion. You suggest there could be exceptions to the rule, but they would still only be exceptions in a dull, conformist region. Sure, you could make an interesting expansion pack out of the East, but really,the devs made the right choice by limiting our exposure to them in the main game.
My problem is, then, why are they included at all, as a main faction that you can support?
Why is the game not then centered on helping Mr. House or the NCR?
Adding the Legion in, without providing an equal amount of playtime that consists of more than just "OMG, another freaking NCR outpost to destroy", is just kind of half-assing it. I do realize that the game got pushed out a bit unpolished, and there might have been more planned, but if there is seriously way more play-time available for an anti-Legion character, then there should be some warning early in the game that there's nothing waiting for me to enjoy in the Great Hidden Beyond of the game if I decide to support them too early on or tick off the NCR very quickly (as can be the case if you foolishly and stupidly back the Powder Gangers before even reaching level 5). It's a very easy way to trash a gaming experience.
Don't get me wrong, I like cutting my way through swaths of my enemies and watching them run in terror. However, I don't want that to be the entirity of my gaming experience. If my only option after ticking off the NCR is to go straight to the main storyline since almost all side-quests get thrown out the window, that's not a good game design. (( A lot of quests I've run into so far require you to interact with NCR, enter NCR territory, or deal with towns where the NCR is directly stationed over or includes NCR-loving inhabitants. ))
All that said.... in response to a "conformist" society, I don't think anyone found Fallout 3 to be really boring, and it's almost a confirmist society by comparison. Take away the very few Mr. Handy's that were around, and most of the robots in the game were out to rip off your face. Super Mutants? You were lunch. Ghouls? Generally reviled, except for Underworld... and areas like that would naturally continue to persist in Legion territory, albeit under the radar.