Riot is not banning you to force you to buy their products multiple times.
Yes they are banning you for money, but it's not in the way that you expect. (they also are banning you for moral reasons but that's a larger gray area) A very large portion of the playerbase is not a fan of toxicity in the game, so if they had to choose between a moderated game and an unmoderated game, they would choose the moderated one. For example (made up numbers) LoL does not moderate, Smite does. A person who is new to MOBAs tries out league and runs into games where 8/9 people are toxic. This really puts them off the game but they like the MOBA style so they try Smite instead. They learn that Smite only has 2/9 people being toxic because of their moderation. This is unacceptable to Riot because they just lost a possible customer.
What some people of questionable intelligence think is that Riot bans you to force you to pay for the same content twice, this is just simply not true. They might say "Riot bans too many people too frequently", but if you play the game you know this isn't true. Way more people should be punished then currently are. Mistakes do happen. People who shouldn't be banned end up being banned. Of course they aren't even forcing you to pay for the same content. All they are doing is forcing you do adhere to basic rules in order to play their game.