So you think people shouldn't follow the rules? That makes no sense. If you break the rules you are going to get punished. It's as simple as that, works just the same way as in real life! Also note that people don't get ban for "nothing", you need to actually break the rules in order for the system to even understand and give out bans.
People like you seem to think people go after anyone and just hit ban. It's never worked like that so I don't see how people can say they ban for "nothing". With that said trash talking is a little different. There are some lines you don't cross, period. I do agree that some things people say should be taken lightly, but it's Riot games and their rules. Plus you need to have several reports hit at you for toxic behavior. It's not like you call one person bad in one game and that's a ground for a perma. It all builds up, people are given numerous chances and if they fail to learn from their mistakes the only person they can blame is themself!
All in all I think the system for now is doing ok, it can be better but it's never going to perfect like how some people want it to be, it's impossible.