Because they’re changing the rules with the new EULA. Under contract law it would override the common law principles of copyright law. It’s greasy. Going forward a modder wouldn’t be able to make a mod for a blizzard game without fear of Blizzard pulling the rug out from under their feet and saying “sorry, this is ours now, you should have read the fine print in the EULA first and maybe thought about making your mod for a different game”.
Is this move legal? Probably. Is it ethical? I argue it’s not. Could this become a disturbing new trend if other game companies follow suit? Absolutely.