Well, I am f2p. But anyway, to add on, I do think that there is a highly disproportionate amount of 6*s when you look at helper lists. I doubt that most players spend money to buy 6*s.

I think it's more likely that the system favors putting people with 6*s on helper lists, so that players see that everyone else has a 6*, so they get more tempted to get one themselves.