This is why I don't do random groom's cakes http://www.acaketorememberva.blogspot.com/2010/02/why-i-dont-do-random-grooms-cakes.html
The OP wasn't wrong to provide a cake. IF there was a sole-provider clause, and nobody knows whether that was the case, it's the bride's problem.
I'm surprised at Jason too, though. I can see your perspective, but the bride wasn't forced to book with someone who had a sole-provider clause, so that's her problem. If you know about it you shouldn't encourage her to violate that, because the wedding cake baker might be the person who WOULD walk away with the cake if they saw another cake there! It's like the people who move into a neighborhood with a neighborhood association then complain about the rules they have. Nobody forces you to move in and sign the agreement, so don't complain when you're expected to abide by the details of what you signed.
I had one groom who kept emailing me about making him a groom's cake when I wasn't doing the wedding cake. I told him I'd do it at the rehearsal but not the reception, and told him the reasons. He agreed, then after three or four emails he let slip "We have no intention of serving it at the rehearsal, we're going to take it to the reception." Okay, dumbass, then I'm not doing the cake. He got all offended when I told him that I wouldn't do it. Too bad, don't try to put one over on me then tell me about it, genius, I don't play that.