There is some reasoning behind it, yes. The final call on the exact amount of time came from the department head though. I'll do my best to explain the basis for the policy;
Our forum system has evolved over time from a very, very basic setup. This setup had little moderation tracking and therefore very little accountability. We've been striving to bring it up to a modern standard in line with such forum systems as vBulliten and Invision Power Board. We're getting there, but still have a long way to to go.
Since we're still not quite there with moderation/editing tracking, we have to have a means to keep users accountable for what they say. Basically, if a user comes along and replies with a really mean nasty comment, to which 20 people reply to, and that user changes (edits) his mean nasty comment to be lovely and friendly, then the 20 replies beneath it are then skewed and the user has effectively gotten away with being mean and nasty with no accountability.
Moving ahead, what I would personally like to see is the ability for users to always be able to edit their replies. This would require better tracking and better tools for moderators and admins to see what was changed and by who. This particular feature happens to be high on my todo list for the forums.
So that's the bare bones reasoning behind it. It's a pain, I know. But it's a temporary (read: hopefully) stopgap at the moment to allow people to edit, but also enforce some amount of accountability.
*** If anyone would like to continue to discuss the issue of the time limit on editing replies, please start a new post and I will be happy to participate in that disucssion. Please don't reply in this post, on the issue of the reply editing time limit ***