You do own the power you purchase. You just decide to give it away to our collective pool of entropy right away. You could store it in a battery instead if you wanted.
And taking credit for something you didn't do may or may not be some form of misrepresentation/fraud, but I doubt many jurisdictions would consider it literal theft.
I know you're joking, but at least in Germany you couldn't _steal_ electricity because electricity is not a _thing_ (according to German law)[0]. They fixed that well over a century ago by just making electricity theft itself a specific crime though.