What made Help wanted and now Classifieds so different that it requires much stricter rules?
Well essentially they are very different from what the other forums cater to.
Forums such as "For Beginners", "Game Design" and "Business and Law" are forums that cater to people who are seeking help, advice or feedback with a specific problem(s) or issue(s). Oftentimes the OP may not know how to describe the issue clearly as they themselves don't know the right words to use or have the knowledge to make directed questions. Moreover the very different natures of the forums mentioned might result in one forum being populated with code dumps, another forum with art, and another populated by bullet points. You might argue that we could apply strict formatting rules to these sections but it would in my opinion be detrimental to the health of the forums and the community and an unnecessary barrier to the many initial posters who turn up to ask a question and then disappear into the wilds never to be seen again.
Job advertisements tend to follow pro forma though - this also applies in the world. A person reading a job advertisement would have a reasonable expectation to see the Name of the Employer, the job title, a description of works (this may also come as an attachment or in person if informal recruitment), a remuneration description and a contact. Special conditions such as location of employment may also be included. Recruitment ads can definitely come in different styles - but the information is normally common to all.
As to other reason's (i.e. why Gamedev does this) I can only refer to Khawk's post above.
At times, we spent more time and effort policing the formatting and rules than having some form of discussion about the projects and offers.
This point I do agree with though. Not for the paid job advertisements in the Classifieds (which imo is appropriate) but rather the current Hobby Projects Classifieds Forum. Its current implementation has enabled interested parties to engage with the OP. I do note though, not many people take that opportunity up though.