It’s your system

Evil techcorp's servers (hosting online services I send requests containing data to) are mine? Cool! How do I sell those?

Or are we referring to local software that gets & sends my data without authorization?

you either accept it or don’t get to use what you bought

Claiming that's theft seems like (taking artistic license with the word steal to express) wanting an agreement that wasn't offered. Like

How dare evil techcorp make a service I want to use with voluntary conditions I don't want? That's stealing!

I don't think computer hardware typically has those types of agreements, and I can change the software & choose online services.