... whether it's a crime or not depends entirely on the laws in the area in question. There's circumstances in the US where the noted refusal could result in a potential civil case, but it's very niche and has absolutely nothing to do with the convictions of the people involved and absolutely everything to do with contract law and the violation thereof.* I don't know enough (read: I know basically nothing) about that place's law structure, so. Yeah.
I think it was fairly stupid from a business perspective to refuse, though. Money's money, and in this case the amount of influence a single cake can have is basically nil. And the thought that production of something == support of the thing's message is vaguely farcical, imo. Generally, when you've been contracted to do something, you're absolved from most association with the thing afterwards -- responsibility lies primarily with the contractor, not the contractee. Obviously, there's exceptions to that, but they're exceptions that prove the rule and tend to involve grievous bodily harm, not... cake.
But... coming from an american perspective, it's a silly thing. The owner has a right to refuse business on any grounds over here, in most industries. Just as people have equal right to protest, boycott, etc. They're also perfectly able to attempt civil suit against the businesses, they'd just, y'know, fail. But they generally can't really demand folks enter contract with (i.e. sell something to) them.
*Though that does change in certain industries. Public freight, ferex, has no right of refusal, iirc. Package could be wrapped in tear-outs from a homosexual porn magazine and they'd still be legally obligated to transport it, regardless of the convictions of the workers or owners.