Even if what they did is illegal and they get punished, the fines are likely to be something like <1% of yearly revenue and basically a rounding error for them so what incentive do they have to stop doing it?
It makes them look petty, it will add friction to their relationship with the unions, and it probably costs them a little extra to deliberately exclude a relatively small number of locations from the ability to tip than it would to just universally roll it out.
It shouldn't take much of a fine to convince them to give up on it. The bad PR alone would probably be enough.
I'm not sure about that. Plenty of companies receive fines for doing shady and illegal things and I don't think their bottom lines change much as a result.
Caring about looking petty has never once crossed Starbuck's radar. Union and non-union stores look the same to customers (except now some allow credit card tipping), so customers would only know about this at all if they're in tune with this type of news.