While trying to avoid any political issues (either expressed in the article or other views) - what do you think about stiffing someone part of a tip for a political message or viewpoint expressed as an article of clothing? Based on what we know of the story, nothing the server said directly to the customers resulted in her docked tip - just the message on her t-shirt.
If you saw a server wearing a shirt with a view that was contrary to yours - would you do the same? Would you leave? Would you suck it up as an "experience" but avoid the place in future if you got a strong vibe that it was a place that encouraged those "contrary" views?