While I think we all agree it seems insane to join them, and even Sawyer admits they fucked that up, it probably WOULD make more sense if you lived in a post-apocalyptic wasteland where the act of walking to the local store was as likely to get you killed as not. It's the good old "order versus chaos" thing (or in this case I guess order versus attempted democratic order drowning in lack of supplies and bureaucracy) but they didn't have the money/time to sell it. Still, if you use your imagination a bit and focus on the rare comments we do have about the Legion lands behind the army being peaceful and clean then you can just about squint and have it make sense. Kinda. Okay not really, but you can get close. Maybe.
There are a few scattered mentions in dialogue that the Legion does establish peace and security in the territory they control, since all tribals and bandits they encounter are destroyed or incorporated into the Legion. However, the game fails to actually
show the player any territory under the Legion's control, aside from its forward military base. Thus, the player gains knowledge of the Legion through its military activities, including crucifixion and enslavement, without ever experiencing civilian life in Legion territory. Further, the New California Republic can make the same claim that, despite its drawbacks, peace and security are established in its ever-expanding territory.
The concept of the Legion is also founded on a massive error re history, and given its founders in-game motivations (copying a society that thrived under similar challenges instead of trying to recreate democracy from a more civilised era) it s seems to be Josh's mistake more than the character's:
He's basing it on the Western Roman Empire, and seemingly the late ultra-militarized Empire (ie post Diocletion reforms just without the Christianity). Sure, that marks the territorial peak, but that's exactly the kind of mistake that in game Caesar should have known to avoid. Hell, that ultra-militarised despotic era of the Empire wasn't even territorial peak anymore - it's what the West half of the empire was like in the century before its collapse.
Economically, things started decaying in the West once the Republic ended. The idea that West Rome over expanded due to overmilitarisation (going into England was particularly idiotic), which in turn resulted from the fall of the Roman REPUBLIC isn't some arcane idea - it's been the main view for centuries, perhaps even millennia (it was a pretty common view even during the Empire itself, especially as the Empire's early glory days under Julius and Augustus was quite deliberately still a semi-republic, with powerful senates - it was intended to just be a strengthening of the executive branch, the rest of the shift was more of a slow collapse of west Roman institutions until eventually they were so hollowed out that going all out military was the only way to delay total collapse for a century.
Then at the same time as the Western Empire rotted, the East was booming. Anyone in 500AD Constantinople would have laughed if you told them the Roman Empire had collapsed - "what, because we managed to jettison that backwater to the West? Who'd want to live in Rome when we've got Constantinople?" (they actually retook nearly all of the western territories at one point, and abandoned it upon realising "fuck, none of these places are worth anything economically, why would we want any of it?").
Ie NOBODY would pick despot-era West Roman Empire as their "great system" to take from that era - they'd copy either the Republic, or go hardcore monotheist unity and copy the Eastern Empire (no "barbarians" causing disunity in the East Empire, or different citizen classes based on tribe - they convert and then they're all fellow Christians uniting against the heathen hordes).
Either choice would eliminate the "why the hell would i support these guys", problem. Basing it on despot era west Empire is retarded - that's a system of collapse and decline, so of course all the player has to go on are these ungrounded assertions of superiority that don't make any sense in game or out.
