Presumably the employees in unionized stores got some benefits from the union (higher wages, stability? Otherwise what's the point of the union.) So they must already have some baseline they negotiated and have been happy with.
Starbucks is rolling out a different model to it's not structured employee situations, this is both their right since the rigidity of the union isn't there, and perhaps it's needed there more.
As a layperson that "not relying on tips" is a goal that a union should provide (else what's the point?) So it's weird that they now want those too.
Totally get why the union doesn't like it but it's kinda one sided. You are either married or you okay the field.