The 75 character minimum was likely instituted as a way to prevent people from spamming their signature with a lot of insubstantial posts. It was probably not anticipated that people would try to cheat the system (or maybe it was and the parties didn't do anything to prevent this).

I think it is interesting that bitmixer has almost the exact same rules, yet they have not encountered similar problems. People have complained that they were "unfairly" banned/removed from their campaign so it should be clear that someone is watching in the background even though they are not active in the forum.