I suppose it could be assumed that a song under 1:30 isn't going to have the necessary amount of expansion needed to get on the site in the first place, but the thing is the guidelines say nothing about expansion of the source material either. It only mentions "contributions, modifications, and enhancements," none of which would necessarily affect length. Short submissions are rare, yes, but there are enough, and I feel it is a disservice to tell remixers that their mix is sweet but too short when no mention of length is mentioned in the standards. (I have a similar beef about decisions mentioning that a piece doesn't "build up" or "go anywhere" when the standards make no mention of these requirements).
The same kinda applies to bitrate. It's lame to tell someone his song is awesome but encoded too low. Of course on those occasions you'll ask for a re-encode, but chances are by that time he's deleted the project file and WAV already. Why not just tell them to encode at 128kbps in the first place? You've already got a max bitrate...how is it different to set a minimum bitrate? With exceptions made for long songs, of course.