But, in general, this shouldn't even be the point. You should always assume that leaving a tip for someone in a position to receive it is a worthwhile gesture that means a significant amount!
I don't even know how this is so contentious to people honestly. Why make such a point about not doing something nice and relatively small? Your gonna spend $15 bucks on an IPA anyway, why not just help someone out too? Are you worried the bartender is going to get paid too much?
Tipping has become contentious lately because it's been asserting itself as an opt-out part of every Square checkout flow, even in situations where we never used to tip like fast casual order-at-the-counter. In the era of Uber we're also frequently asked to pick a tip before we even receive the service, which feels more like extortion than "something nice". Tipping is becoming more and more contentious because tipping is changing, and the version you describe is quickly becoming the minority case.
But the situation here isn't a clear-cut case like a sit-down restaurant where tipping is expected. If I were handed a receipt like the one in the article for my family's tickets and food, it wouldn't occur to me for a second that the barkeep in the room next door was relying on me tipping on this popcorn purchase in order to make a decent wage on this kids movie night.