I think this is what professors/teachers tend to forget; get up in arms about your salary, don’t whine about your side hustle income. If you would be working for a corporate, it could well be that all that belongs to the company; it is about the subject of your job, it gets used at your job; it is your job, why do you believe you should be compensated extra for it?

I am from a time and country where syllabi were written by the professors and were free with the course (you paid a few cents for the printing/paper). Those were thick stacks of paper and they were free. Sure it could’ve been better; they could’ve been free as in speech, but at least they were free as in beer. That is your job and you already get paid for it.