Practically, the reason for the restriction is to make sure that the Masters are actually fulfilling their role of "proponents and exemplars of their chosen element". Each PvP event has unique rules, but all of them share the quality of having an aspect that specifically nudges the Masters into their role. The Master's Competition was very strict, the proposed War rules were less so, and this event is the least strict of all.
In fact, Terroking just ninja'd me and said exactly that.
PuppyChow is showing the players what a Time mark is good for. That his deck happens to be mono-Earth is irrelevant to the fact that, without Time, that deck wouldn't be possible. That's representing Time, straight up.
You can continue to look at the rule as a restriction, or you can see it for what it is: a rare opportunity to display what your element can do in a supporting role instead of as the main player.