"professional behavior" would make sense if both sides are getting paid.
If only the user is getting paid, and he wants support or something else from the FOSS author, why should the FOSS author be held to this "professional behavior" standard?
No, clearly not. Enforcement of behavior is a terrible thing.
And of course it won't stop with a few, fairly reasonable suggestions. It never, ever does.
A very thin end of a wedge is stil the end of a wedge.