Then what if badspot uploaded them?
I don't know what Steam's T&C's for Workshop is or what their personal rules are on who they approach for trying to add things in, but I don't think they would let you create promotional items for TF2 and add them in for free.
When you see new games arriving on Steam and they do a special pre-order bonus where you recieve a promo TF2-Item based on the game you're purchasing, that has gone through a deal in a different way to Workshop.
In that situation the owner of the game has asked Steam if they could do a promotional deal with them for TF2 items. In that scenario Steam would ask that the game owner pays them for that promo.
With Workshop it is different. Steam chooses what stuff they put in. And they won't put promotional stuff in and pay you for it.
Because otherwise, what does Steam get out of this? They have to pay you for it and then give you the chance to have promotional items in TF2.
Workshop is something that is used by regular fans to make regularly interesting items that aren't promotional in any way. This is a deal, but not promotional, so Steam gets the most out of it (a new item for them to sell in-game) and so they pay you money for it.
Promotional Items are used by companies selling a game on Steam in order to promote their own game. This is a business deal and the game company gets the most out of it, so they pay Steam money. And Steam gives the item for free for pre-ordering. They can't sell the item in their in-game market.