How come some non-donors can post in Comic Showcase?

Because we're special.

By that, boinky means that these users gave a "special" donation to Brad.

Also, they're our henchmen.

I gave Brad multiple special donations.

I'm guessing that it's from when the forums were invite only, some non-donors were invited to the forums and so when the forums were re-opened to everyone they remained flagged with full access.

I could be talking pish though.

That would be the exact reason.