Why didn’t they just change the rules?

“Having a supervisor around” isn’t necessarily a requirement from the union.

From what I understand talking to the staff, it was because none of the staff (at the time, who weren't part of the original unionization push) actually cared enough about the union to do all of the union stuff (like going through the CBA finely and and renegotiating the rules, regular meetings, shop stewards, all of that). They all had a mentality of "this is a coffee shop I'm going to work at for a year or two, not something that I want to put any extra time into beyond the shifts I need to work to pay my way through school"

Edit: If I'm remembering right, the thing that finally sealed the deal for them was that Starbucks corporate was offering better benefits to the entire company than the union (which was made up of like 10 people) had negotiated for themselves.

Starbucks told them it was because the union