Peen size is competitive.

Peen you own shows up on the Leaderboards page.

On top of the options to buy illegal enhancements is clearly Posted:

Purchasing illegal enhancements will disqualify you from the competitive aspects of Forumwarz.

Furthermore the big page full of warnings when you buy one Posted:

However, to keep a level playing field with those who play Forumwarz competitively, buying this item will disqualify you from the competitive aspects of the game, specifically Forumwarz Domination and the Leaderboards.

So that’s why.