You're basically giving permission to everyone to use your creations as they please once you upload them to the gallery. Any simmer is free to download your creation and place it in their game, edit it, feed your sims to the cowplant, paint your house pink, etc.
Once uploaded to the gallery, our creations stop being ours and belong to EA.
Sure, everybody would like to be recognised and credited for their own creations. However, you must remember that this isn't a popularity contest. Don't make a big deal about it. The only thing that matters is that you're happy with your creations and are able to enjoy playing with them. Besides, I bet that most of these "thieves" are just kids reuploading stuff they like to the gallery. I doubt they're doing this with ill intent.
