Isn't this stealing resources from the owner without their permission?

Just like the person that sued a company for sending them unsolicited facsimile, claiming they used his electricity and paper without permission - thereby stealing a resources and wasting them.

This is where the "opt-out" idea flipped to "opt-in" in the UK and elsewhere.

Although, if you've blindly opted-in then more fool you - but surely this behavior is an obvious "opt-in" not an "opt-out" scenario.