I think the bar for being "a giant piece of shit" should probably be a little higher than the act of getting a coffee. I mean, if customers are giant pieces of shit, what are the Starbucks executives? Galactic boulders of excrement?

Jokes aside and more practically speaking, I think it's more productive to urge people to consider patronizing their local cafes - or learning how to make good coffee at home - instead of trying to shame people who probably don't know or honestly care that much about random labor disputes for getting a coffee. Beyond being more positive, it's less likely to annoy people into spite.