Kickstarter could give a bonus to everyone who does not join the strike, then fire everyone else and replace them.
Sure, it could end up being more costly, but there are options.
I have a feeling that a company like Kickstarter making a move like that isn't just "costly". Seems more like mutually assured destruction. The PR fallout of a move like that is going to cause some serious hiring problems going forward. Plus the loss of domain experience alone would be massive.
The labor laws prevent them from doing that. It's this lack of contract liberty for one side of the negotiations between unions and employers what makes the effect of unions harmful.