It's because it's so small that the devs thought that things with 3 or less letters would fit into almost everything. I don't know what they were thinking.
I talked to Simon about it recently, and he said it's a limit on whatever he uses to do searches (I forgot now). It only allows searches if at least 4 characters. So, even if he wanted 3 letter searches, he can't. I can probably delete those tags from the list, and disallow 3 letter tags in the game now
Also, 3 letter searches were actually planned for tpt++