Honest breakdown: It's to make their repeat-offense and honor statistics look better. This isn't about actually improving the community, but rather about inflating the number of 'honorable' accounts. They don't want you to grind your way to 'redemption', hence making that process continually harder at every turn. The goal is for you, the dishonored player, to create a new account that displays better behavior- so they can exclude the old one from stats due to inactivity and brag about how much better their community is getting. If you're someone who just fell victim to their bots, you know better than to use the in-game chat now, even with friends.
If you're an actual troll, you've learned a few tricks about upsetting other players without triggering discipline. Either way, they get to count you as an 'honorable' player, and your old account becomes part of their non-recidivism propaganda (<fake>% of players who are disciplined never relapse, yadda yadda). Riot doesn't care who the actual victim is in a situation, or about you as a person.
Riot doesn't actually care that sub-Challenger games are being ruined. Riot cares about their player behavior statistics, because improving those helps make them more palpable to media, which in turn makes it easier to market their e-sports, merch, etc. (These things are the bulk of their money.) Riot honestly probably resents the bottom 99% of all its players- we don't make them much, if any money. The players who are entertaining to watch are their cash cow.
