My gut is that there's some difficulty with calling it generic theft. States have been passing laws to define the unauthorized use of online services. There's quite a bit of analysis online and the consensus seems to be that accessing an online service with someone else's credential for otherwise legal purposes (e.g. simply streaming content) has not been treated as a crime.
If it was so obviously and fundamentally theft under state law, I think much of this lawmaking and analysis would be unnecessary.