I don't recall the original source saying that BK's TAship was in jeopardy. Of course, it may have been, the prof may have said so, or maybe BK just feared that his position at WSU was at risk.
In any event, essentially reprimanding--even if it wasn't stated as such, it was a public humiliation--an employee in front of 150 colleagues, clients or however one thinks of undergrads, was cruel.
BUT, as someone with years of experience as a TA and as a professor supervising TAs, the degree to which student evaluations rule universities is both astonishing and appalling! (And I did my grad work and teaching at one of the most prestigious universities in the USA; I also, FTR, had very high numbers, which is how I kept my job.) Treating students as customers instead of protégées has had--IMHO--a deleterious impact on higher education everywhere!
Even though, in my experience, TAs are evaluated separately, students' feelings about TAs tend to spill over into evaluations of the professor. If s/he was getting complaints from scores, dozens or even just tens of students, I don't wonder that the prof was very concerned (which explains but does not excuse the solution s/he chose).
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BTW, technically, a TA ("teaching assistant") usually holds classes of his/her own with smaller groups of students. (E.g., when I taught a class of 120-150, I had four TAs, each of whom taught 3 one-hour small group discussions per week.) A TA may also grade papers, run the AV equipment, etc., whatever the prof requires. (There are cases of abuse where TAs are treated as personal assistants, but this is very much against policy.)
A grad student who merely grades papers is often hired as a "Reader". This is true at UCLA, where I spent most of my academic career. I don't know how WSU classifies student employees.