The problem is that "having a union" isn't a bad deal that you can just legally refuse, any more than following health and safety rules, or overtime rules, is a "bad deal" that Starbucks can refuse if they want. Having a union at the store costs Starbucks nothing, or maybe a little bit of the manager's time here and there.

An individual contract might not be financially viable for a given Starbucks location, but nobody is arguing that they will be forced to sign such a contract.