I wonder why someone would cheat to earn ranking points, but it occurs. People on chess sites use computer programs to artificially boost their rating. Why it helps their self-esteem to be a data entry clerk for a computer program is beyond me. But unless I'm playing for money, so what? I don't feel it's necessary to worry about whether, on a free site where there's no tangible reward for cheating, someone is cheating.
If someone chooses to cheat, that's their bad decision. They don't get to enjoy the real fruits of success if they win, and if they lose, that probably makes them feel particularly stupid.
More importantly, given that there is value in a community without cheaters, it is easy to catch most cheaters by analyzing the data. How likely is someone else to be present in my game, especially an anonymous game? Do that person and I tend to do better on average when we play together when we do not? Straightforward data analysis can identify any unusual correlations. Humans can investigate pretty easily whether there's something fishy going on.
Finally, why not have a player rating system for integrity? I should be able to flag opponents for suspicious behavior in a private manner. This could be taken further such that I can ban a player from my games - I can't join games in which they're already a player, and if they join a game I'm in, I automatically withdraw.