I am not prepared to argue the details of the barista labor market but SBUX has a reputation for taking good care of their employees, compared to other places that hire that type of talent (401k, tuition coverage, etc.) Meaning, it expects employees to work hard/go above and beyond and tries to do the same for them - at least in their own assessment and I bet many employees agree.
The employees who chose to unionize presumably didn't like what the company was offering them on its own and decided to lever up their demands. That changes the relationship from "taking good care of each other" to bounds by rigid rules. Once you bind me by rigid rules, I am not giving you anything extra (if for no other reason as I can use that as leverage in our next negotiation which is what you're forcing me to do)
I take issue with this framing. This has some implication of a malicious, or rent-seeking, third party, interfering with SBUX and its employees. Except that it's a union -of- employees.
And the union wants a better/equal salary. But SBUX wants to pay a tip-dependent wage and then remove tipping options from those stores. You can't say, objectively, "Well, the union wants everyone paid equally/fairly, and they've got that now, what's the big deal?" when the wages being paid equally/fairly are predicated on tip income.
By your logic, if a restaurant workers union wanted to have everyone on equal wages and remove tipping from the equation, they should be happy to have tips removed and have everyone making $2.13/hr (well, it'd have to be $7.25/hr) because hey, it's equal, and all they need to do now is negotiate that rate up, right?