It's always been in the rules. From point two on the rules list for entries:
"You may not obtain permission from other artists to incorporate their work into
your own submission. Everything submitted, every element must be entirely the
creation of the submitter." (emphasis added)
The reason is that you're all competing as individuals in these events, teams are more of a "for fun" thing with the possibility of winning some extra prizes at the end. You can work together in that one person may make an icon set, another a matching WB, and another a matching wallpaper etc...
