Many good reasons. As a former upload you have to take my word for it.

-Fewer shields/swords uploaded = FASTER for us to upload. Supply and demand. Less people will wanna pay for it so you filter others out.
-If you're gonna pay for it you're gonna do it right. We get so much crap shields that we can't upload because they don't follow the rules. The system makes everything consistent and if you do suck, we can just reject it when we get to it. Before it was all email and we all know how thousands of shields can be unorganized in an inbox...
-Keep the game alive. Some people don't buy gralat packs but love shield making so there's a different market. A lot of people complain about the game ripping off people, but bills need to be paid. Eurocenter does way too much work for little money. I am surprised the game got this far. No one will ever believe that but try not living with your parents first ;-)
-if people are gonna copy you, they gonna have to pay for it, so you can't feel that mad bwahaha