I am not saying I agree with it, because situations like this make it clearly too limiting of consumers who get stuck in this type of darned if you do/don't mess...
In the interest of the info itself though I believe they originally started preventing users from loading older ios versions because people would upgrade to a newer version that could not be jailbroken, realize what they'd done, and then go back to a version that could be jailbroken. This was even more effective when the phone started going nuts and needed to be backed up and reloaded...and the only option was a new os that would not jailbreak. In reality the jailbreaks have come out so quickly that it has not been an issue for many.
Last I checked, the only technical reason you can't go back to an older version is because there is a type of activation code itunes looks up when doing it, and apple stops providing the codes when a new os comes out. You can back up the code you have before a new os comes out and then run an app locally that pretends to be the code server, and still go back to an older version. This is true even if your phone was never jailbroken and I don't think they'd ever be able to tell. You just have to do the backup before the current version stops getting activated.
They may have added more blockades since then though...and I wouldn't know because: The preceding information has been brought to you by me getting tired of it all and selling my 3gs and getting an andriod, which has been easier to mess with so far.