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So you think it is fine from a competition perspective to have an addon manager like WoWUp that supports all competing platforms, while Curseforge isn't able to do the same?
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Of course Blizzard has every legal right in this case, at least in the US and EU.
If you want to use their tools and API for their game, you have to agree to their conditions. And them requiring you to make all addons you make free of charge (and also: open source) is a perfectly fine requirement (not all conditions are legal, but those are). Many companies have been successfully sued in the past for breaking similar licenses, like how Linksys once broke the GNU GPL license by using Linux.