I'm shocked that I have to clearify this

Today someone asked me to make a nsfw comic for them for free and called it "collaboration", and not "commission" just because it's their idea and their plot, therefore a "collaboration" should not be payed for. I kindly said no and they started being cocky about it, so I explained why my work costs money (shocking!), so they threatened to sue me for making profit off Hasbro's characters (which I'm not, it's fanart and I'm not payed for it.)
Just to make it clear - the only time I'll be drawing free stuff is when I'm opening art requests. And even then I don't promise that your request will be drawn. I'm here, a professional artist doing my job. And that job should be respected, and me being a person should also be respected. I respect my auditory, and I hope you guys are all adequate people here, so we can have fun together.