They don’t have to spend it all, you know. If they can reward corporations for negotiating in good faith by limiting the damage of the strike, they’re retaining leverage. They can still go full strike later, but it looks like it’s not necessary.
I’m sure the UAW knows what they’re doing.
Besides, if they were to hurt the corporations too much, they’ll end up hurting sales of American cars, giving foreign competitors inroads. China’s car workers aren’t unionised, so importing those cars hurts unions everywhere.