Because trolling is disruptive to the community as a whole, bestiality is disruptive only to the poor nameless animal that was raped.
Pretty sure people have been banned for admitting to trolling in other places outside the Blockland community. That doesn't have any direct impact on us.
Besides that, as Badspots comic nicely illustrated the situation, we don't actually have any proof that what Pie Crust is saying is even honest, we can't punish him for him saying stupid stuff unless that stupid stuff somehow disrupts the forums as a whole.
Why would he need to prove his own confession? Public scorn isn't an achievement to aspire toward, and even if it was, it's still not something that should be withheld just because he wants it. You wouldn't let a Flash Mober go free just to spite him when he said he did it for attention.