Look at it the other way. Suppose they honestly believe that $15 is the right minimum wage. If they just raise it to $15 for their workers, why should they let other companies freeload?

Look at other sorts of pro-worker laws, like safety. If Amazon were lobbying for increased OSHA oversight of warehouses because they didn't want to compete with companies who treated worker bodies as disposable, would that be bad? I don't think so.