Does their argument gets invalidated if they don't verify *every* restaurant ever? Nobody has the time nor the resources to follow their moral standards with 100% precision, but if we're doing our best I'd argue we can still take that moral stance.
Recently a slave labour scheme was dismantled in my country in which some wineries were keeping slaves to produce grape juice. The companies were on the news, and although I do love some grape juice I will never ever buy from them again. Do I check *every* single source of the products I consume? Of course no. Can they eventually do some marketing tricks and fool me into buying from them again? Maybe. But I do my best and I feel like this is sufficient to claim this is a good moral stance nonetheless.