The logic is I bribe you, you bribe me, we edge out other competitors who are smart enough to avoid unions. From my reading of history, Honda came THIS close to letting the union in when they first started manufacturing in the USA. It was one guy who resisted it and cleared the way. One man.Personally, I don't like the union provision in the tax credit. There's simply no logical basis to justify that. US manufacturing? I'm all for it. US-based companies (which is not in the proposed law but I wish it were)? All for it. But not union-built.
That said, I see BlackGTP's point. The argument quoted above assumes there's a level playing field to begin with. That's not the case. Korea is subsidizing Hyundai and Germany is subsidizing VW and so on (through government-funded healthcare etc., not to mention more overt ways in some cases), which means GM, Ford et.al. are already at a disadvantage. Still, I would have preferred if that was addressed through other means than by relying on unions as the determining factor.