I suspect they felt that taking a very hard line would send a message to the union. Instead the message that they sent was to dig in your heels.
I suspect they don't want to strike, but they could be pushed into a corner, and if that's the case they have considerable leverage,
The corporation knows the sensitivity of these kinds of issues, they have a lot of experience with what will provoke a local strike. What I suspect they (the GM management) didn't anticipate is that it would reach this intensity.