I think it's the same reason most games don't allow selling in-game items for real money. They can go 100x the original prices. (Of course, even in those cases people have found workarounds.) If that happened with the first NFT they sell, this can look very bad for Square Enix.
But the problem here, is that NFT users would expect themselves to be able to sell it. That layer of ownership is one of the differences between NFTs and centralized databases.
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It just seems if you buy something, it should be yours to sell if you want...
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