invitation vs advertisement

I'm taking a class by phone and we're communicating by email. Today I got an email to the class list advertising a vaguely related paid class (enticing language, testimonials, the works) from one of the students. Oh yeah, she's the primary contact for this paid class.

I sent an email saying it's rude to advertise to a class list without consent. I got back a nice enough email saying she wouldn't do it again, but she called her email an "invitation" several times.

I find myself even angrier than I was originally. It's one thing to spam people, and it's another thing to pretend it's an invitation even after you get called out for spamming. Argh! As usual, I get most upset when people try to warp reality.

I wrote back suggesting a language change. "You invite friends. You advertise a paid class." I notice that something in me is afraid of a backlash for pressing the point.