The custom filter is a good explanation. It's the only one I can think of actually. Great firewall appart...
Anybody has feelings, or even better, statistics on the type of transactions included in their blocks, in particular on their category (free or not), and the fee per KB (for the non-free), and their priority?
I think I saw something along that lines recently, don't remember if it was here or the speculation thread, or about AM at all.
EDIT: That would make a good official question for friedcat! I'm really curious about these filter policies and their motivations, if they do exist.