It's your system and you agreed to licence your data to them. So technically it's not theft. But also technically, pirating isn't theft either, you're not breaking into microsoft HQ and stealing a product key.

On a practical everyday way, yeah, I would say they are "stealing" your data, since they hide that as a clause in a massive EULA that can be altered at any time, and you either accept it or don't get to use what you bought.

[–] lmmarsano@lemmynsfw.com -2 points 17 hours ago

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.