It's somewhat unfortunate that the ruling merely reasons that it wouldn't be practically possible, rather than reasoning that such a copyright would be a burden on society without any real gain.
If you're more concerned with the unenforceability than the consequences of enforcing a law then something has gone wrong. Going back to your example, a law against hunting sentient creatures would make sense regardless of the fact that establishing sentience is a hard philosophical problem.
When home VCRs became commonplace, creating an automatic copy of a broadcast was still illegal. Some German high court ruled that people's right to privacy in their own homes trumped law enforcement's interest in enforcing copyright, so the law was unenforceable in those cases. Hence was born the exemption for "copies for private use", and the related "tax" applied to sales of VCRs (and later on, blank CDs and DVDs) that was used to pay off author associations.