lol yes you could get around the rules, just like right now you could simply deny that an obvious foul has occurred and force the decision to the shooter. I think you overestimate how enforceable any pool rules are currently. There are no pool police or pool juries or pool prisons, but making a rule change like this could change otherwise honest people's attitudes about this situation, which I believe would improve sportsmanship.
I'm not under the impression that rules make cheaters go away, in the same way that laws don't make criminals go away. They're just a way for a group to explicitly declare how people are supposed to act.
If anyone is overestimating how enforceable the rules are, it's definitely you. You say this rule would make it so honest people would call fouls on themselves. In your example with the rule in place, the shooter committed a foul, and didn't call it on himself. He just sat down. That means that he isn't one of the honest people affected by the rule, because he ignored it. Therefore, in that scenario, the rule is still not enforceable, and doesn't make a difference.