Samana Johann adds that phrase to his posts. I think the reasoning is:
User contributions (i.e. all posts) to Stack Exchange are "licensed under cc by-sa 3.0 with attribution required"; the text of this license includes:
You are free to ... Adapt — remix, transform, and build upon the material
for any purpose, even commercially.
In summary, someone could sell a book consisting of posts made to Stack Exchange, and/or Stack Exchange is or would be allowed to operate this as a commercial site.
Samana Johann (and some monks in general) dislike the idea of Dhamma being sold -- in particular, I think that monks are not allowed to sell Dhamma.
I think I've had some extensive discussion on this subject in the past (probably on Meta and probably with an earlier version of his user ID, if you want to look for it, though possibly in a deleted comment thread). I think that it (posting that disclaimer) is unnecessary and useless and a bit annoying, but (as you can see) I allow it of him.
FYI apparently there a concept in France, défendu mais toléré, "forbidden but tolerated". The example I read was of parking your bicycle in the courtyard of an apartment building: doing that is forbidden; but if only one person is doing it, no harm done, it's tolerated; but if everyone starts doing it, well, there's a rule against it, it's forbidden, they can start to enforce the rule.
Similarly this kind of "signature" is forbidden, and signatures or boilerplate should as you say be in the user profile only.