It seemed pretty clear to me at the time that PZ Myers was trolling — in the classic sense of doing something provocative in order to selectively make people who become outraged look stupid in public.

As ESR puts it (with his usual subcultural defensiveness):

The well-constructed troll is a post that induces lots of newbies and flamers to make themselves look even more clueless than they already do, while subtly conveying to the more savvy and experienced that it is in fact a deliberate troll. If you don't fall for the joke, you get to be in on it.

— http://www.catb.org/jargon/html/T/troll.html

Trolling isn't really an attempt to have a conversation or debate with the trolled. It's an attempt to demonstrate power, status, ability to take control of the social situation; to deflate the ego (or self-control) of the outraged victim, and so on. It is a performance for an audience, in which the victims of the trolling are a not-entirely-consenting part of the performance.

IIRC, the political context of that particular troll had a lot to do with religious folks (not particularly Catholics) insisting that non-religious folks should be obliged to express respect or deference to religious symbols. See also Qur'an-burning, "Everybody Draw Muhammad Day", and a long thread here involving British people being offended by salmon. I argued in that thread that one point of "Everybody Draw Muhammad Day" was akin to that of "Banned Books Week" — it is a show of defiance against those who would censor particular expressions.