> Grading is part of what it means to be a TA,

Checking students' homework is part of what it means to be a TA. Grading is a choice the university makes; it's absence makes one no less of a TA.

> being a TA is not some awful exploitation

It's certainly exploitation. For some TAs, who get overworked and lorded over, and in universities when the pay is low, it's awful exploitation. For most TAs it is 'regular' exploitation of getting a crappy salary for serious highly-skilled work.