I've been on many forums, and I have never seen this either.
To attach ads to a particular post, without the knowledge or consent of the author of that post, seems, to me, sleazy, dishonest, and unethical. It creates the implication of a personal endorsement by the author of any post to which such ads are attached, of the content of those ads. It amounts to a form of identity theft, in my opinion.
I have no problem with appropriate use of ads, to generate revenue to support a site such as this. But to attach them to a user's post in this manner is not appropriate, and I find it highly surprising, and a bit disturbing, that anyone would think that it is, or claim that this is a widespread, accepted practice.