There’s certainly a not unreasonable argument that it is if you agreed to watch ads in exchange for getting something. If you do a project for me and I agree to pay you after I receive your invoice and then stop accepting mail, refuse all emails, ignore all phone calls, run away from you when I see you in person and otherwise do everything possible to prevent myself from receiving your invoice, wouldn’t you consider that theft of services?
That isn’t to say I think all ad blocking is theft of services, nor to say that something like broadcasting with ads constitutes an agreement or even that abuse of ads in such an agreement wouldn’t justify blocking ads. Just that the argument is not unreasonable