Nowhere else in reality does this really occur, so your “of course” here is highly unmerited. The map editor is a tool, what you do with that tool does not entitle the tool maker to the intellectual property, much in the same way that games are built in a game engine, the game engine does not own the intellectual property of the game. This is one of the few things that I think would absolutely not hold up in court.
I’m not Interference, the person you posed the original question to, but the reason why this is sad is because these clauses objectively limit the creativity of a map maker as well reducing motivation to create custom maps not only because your creativity is now limited, but now the tool company is saying it’s owning everything that you make with its tool.
WC3 spawned and attracted many communities through its custom maps, WC3R should be a celebration of what made Warcraft III great, a re-kindling of one of the games from Blizzard’s golden age, instead we see that Blizzard appears to resent the map making community, and wishes to alienate them and the communities that thrived on them.