The rule and change in order formatting (and some other rules) was made in response to people (including one serial offender in particular*) making arbitrary posts to bump their FS post to the top. Heck, I was guilty of this too - posts like "price drop" or "email received". The "problem" is that there's so much traffic on this site that a FS post made in the morning can easily get buried on page 2 or 3 by the evening, and with everybody else bumping their listings, one had to play just as dirty to keep things "fair" to them.
This way is fair to everybody.
It also requires far, far less moderation. Other sites might have a once-per-day or once-per-X-days bump rule, but that means that some poor sap has to constantly check every listing to make sure the seller isn't abusing the system.
*This particular seller, who is banned here, would have upwards of a couple dozen or more listings, and they'd all get bumped at the same time, meaning the first page of FS listings would be almost entirely his listings.
