I think what really irked them was other students found out and started demanding it from all the real professors and adjuncts and in other contexts. Then I was an actual problem not just a troublemaker.
Maybe I'm weird, but this sounds like the highest form of compliment.
It's a patronage network that exploits young people.
And their parents. And the general taxpayer (via student loans).
You end up with books that sell < 100 copies a year that would never be published from a market demand perspective but provide extra $ for the professor (which matters to the Dean for retention and salary).
I graduated from a large state university engineering school 20 years ago and this was the case so I'm sure it's only become more "enshittified".