The Alpha user and Wolfram certainly could share a copyright for nontrivial inputs which represent a _de minimis_ creative contribution, but Wolfram can simply

The Alpha user and Wolfram certainly could share a copyright for nontrivial inputs which represent a _de minimis_ creative contribution, but Wolfram can simply put a copyright assignment in the Terms of Service license agreement the Alpha user agrees to, and I bet if you read carefully through enough legalese, you'll find that somewhere (just in case).