You're assuming the Dominion approached the Breen about joining. It could have been the other way around, with the Breen approaching the Dominion about joining. In that case, there would be no discussion of "not attacking" the Breen, since that wouldn't be an issue.
And remember, a Breen was imprisoned at Camp 371, so there was at least one changeling operating among the Breen. That fact suggests to me that a changeling manipulated the Breen into joining.