Yeah I agree for a humanities/social sciences program, this would have been considered absolutely mortifying. I was in an English program and taught composition on my own as a TA but did act as a TA one semester for a lit professor. He actually asked me about my grading privately before the first paper was returned to them because he felt like I was being too hard on them, but I was not a first-year teacher/grad student and he'd also hand-picked me for the position and trusted me and was a very kind person. He wasn't approaching me to humiliate me. He had a genuine concern. When I pointed out my reasoning for the grades, he ended up agreeing and telling me maybe I was being too lenient. LOLOL
In any event, I think it's possible the professor is famously gruff and didn't think it would be a big deal, but I think it's also quite possible that he was very well aware of how embarrassing it would be for BK and maybe did it intentionally because he felt like he needed to be knocked down a peg or two.
The professor I worked with had talked with me about us being a united front since students will often approach whomever they think will be the softer touch to beg for grades--they always went to him instead of me--so I think even if he'd been very unhappy with my performance, he would never have undermined me like that in front of students. He'd have torn into me in private, though.
I am pretty sure that's a pretty standard dynamic for many professor/TA relationships because otherwise the system breaks down, not only between the professor and TA but also between the students and the TA. I'm assuming this professor likely is very used to having TAs in his class, and it seems significant to me that he basically threw his TA to the wolves. MOO