Let me put this into prospective. If a writer posts that they are looking to get paid for their services, that also is not allowed. Nothing can be monetary on the forums unless someone is actively looking to commission someone.
If we didn't have this rule, then there would be commission posts everywhere. The rule I posted is a screen shot from the actual guidelines.
While I do not necessiarly like the idea of an unpaid collaboration, there is no rules against offering services for free. I see lots of writers looking for collaborations, and again, I do not necessarily agree with it. However, and this has been discussed before on the forums, if a writer puts time and effort into their pitch, then there is no harm.
If an artist wishes to collaborate with a writer or artist as a passion project, then there is no harm.
Ultimately, this is how the rules have been for forever. And a quick edit, there is a reason why we have the collaboration post overall. Ultimately, if you are displeased with the rule, then I would recommend speaking with a moderator and discuss your thoughts about the rule.