Herd mentality is a powerful persuasive tool. If you're not a designated loser, it's much simpler to do nothing and go with the flow, especially when there's a significant imbalance in combat power. In this case, the leader is charismatic and has the sole gun. Most people's instincts are to obey the gun wielder, not challenge him empty handed. And while harsh, they cannot deny that Reito's methods have allowed them to survive thus far.
It's literally how fascists come into power, so this isn't a reflection of current American politics as much as it is textbook fascism. Reito has made his group entirely dependent on him for direction and they're unable to think for themselves. Yes, it is a bit unrealistic how quickly and thoroughly he seems to have done that, but again, not outside the realm of possibility.
On another note, denying the possibility that entire groups of people can be influenced like this is exactly the reason why America (and many other Western countries) are experiencing a huge ultra-conservative wave. Thinking that everyone should be able to resist the easy way out temptation like you are able to, and that if they can't, they're inherently bad or evil people, is exactly what drives those people to band together the way they do.
