I do see many bartenders pouring very foamy beer and pouring lots foam into the drain to make a full glass of beer. I never think the beer tastes over carbonated, but I do wonder how much beer they are wasting.
A lot of breweries struggle with getting Co2 right if they don't know how to check the stone whetting pressure or don't have a way to measure Co2 (such as a Zahm). Bartenders get used to just pouring off the foam. You are right, it's a big waste.
Of course, there's also the issue that some bartenders don't really care how much beer they waste since it isn't their money.
Beertenders are just actors. It's a performance, so that you tip them more. They actually hate their customers (hoo boy, trust me on that). But they put on a good show, and are nice to you so that you feel important...so that you tip them more. It's all performative.
I loathe beertenders because 1) I know how fake they are; 2) they know far less than me about beer, but they pretend to know, and think they know, everything about it--and also they hate you if you try to talk to them about beer because they think you are simply trying to show them up; and 3) they make waaaaaaaaaaaaaaaaaay more money than anyone on the brewing staff. I don't think anyone outside the industry realizes that vapid, uneducated people doing nothing more than pulling a handle to fill a glass can make close to upward of $60-80k a year.
Granted, it's stressful to pull a handle and fill a glass. I get it. And wiping a counter/table with a rag takes pretty much everything ya got.
You are so correct about the whetting pressure thing. Where I worked, we'd simply put our ear to the FV and listen for just the right volume of hisssssssss as we turned on the gas. And then a Zahm-check when it tasted right. Kegs were behind a wall from the taproom and literally just a couple of feet from the taps, so we didn't have to worry much about balancing the lines.