I'm not posting this as a mod - I resigned my moderator-ship(?) a long time ago. The reason why the tag hasn't been removed yet is probably going to be obvious in a second. Jedi will be along soon I'm sure to give an official comment, but this is my understanding;
Anyway, the primary reasoning for now allowing any posts at all outside of the weekly bump is that is becomes too messy and it's the lesser of two evils to just not allow them altogether. The 'Unread Content' feature (that's used by many members) used to be around 60-80% bumps and then a select few bits of meaningful content elsewhere. This was chiefly due to a staggering number of people having discussions in their classifieds threads (I'm talking 4+ posts long) or just assuming the rules on ZeroIn (where a bump can be made every 3 days) was true here also. There was about a 50/50 split on people being reasonable with their threads (replying only to bump reasonably or correct something in the OP) and those who were taking the piss.
That's why the decision was made to more strictly enforce the 'no bumps' and - in particular - to police discussions that could be made via PMs, people saying things like 'is this still available' or 'would you take X' made up a very significant portion of all the posts from a given week on the forums, and that was having an adverse affect on others making their classifieds (and just - by sheer chance - not getting people posting on them, buy rather PMing or taking no interest). More importantly, it was - for a few - getting really annoying that they'd have to pass over so many newly active threads when otherwise posting on the forums with nothing to do with classifieds or just checking in on their phone. These things can be disabled with some effort and a little knowledge of the forum, but it was a small gripe.
As Samurai has said: The warnings are indeed a pre-made affair made to save moderator time. In some cases they can seem harsh I'm sure, but they're intended to be functional. These are rules that - in theory - any user should know. I wasn't aware of anyone (during my tenure at least) reoffending so much that they got a ban (temporary or permanent). The zero-tolerance policy is to save time on the part of the mods and of course avoids any implicit bias on what is and isn't allowed to be posted in these threads. I for one used to enjoy people with vastly overestimated prices on their gear getting shat on by forum regulars, but that not happening anymore is a side-effect of this policy.
So here we come onto the real issue: Some time ago we requested that the classifieds subforums simply not allow posts to be made by anyone other than the original poster (OP). This is the same as ZeroIn's policy (as an example) and forces people into sending PMs. However, this never got sorted and is one in a long string of let-downs from our administrator that were the reason for me resigning. He's not got around to removing the tag though (which should probably indicate how much he actually cares about things like this getting sorted). This is a failing of the forum and I'm sure still is a huge ballache for Jedi (who has to manually review these threads a lot of the time) as much as it is for those who receive warnings and get their posts deleted. Unfortunately there are other occurrences of quality of life improvements like this just not being enacted.
