Check your contract with them.
If you have an SLA that guarantees speed, service availability, etc., and the provider refuses to make good on their end of the contract, then it's breach of contract - not theft.
You know I don't. The only thing my ISP will guarantee is that I will have my service (such as it is) cut off if I don't pay the bill on time.
Not that the concept is worth anything anymore, but there used to be an understanding that the seller would at least attempt to provide what they were charging for. When I buy a toaster, it is implied that the device will actually toast bread, not just occupy space in my kitchen. Imagine printing on the box that this toaster will toast, but how often it will work, and how well it will work are not known and are not the responsibility of the seller of said toaster. That's internet service.
Buy a bus ticket to downtown, and maybe it takes you there, or maybe it takes you to the west side. Or maybe it takes you halfway there. Your ticket price is always the same, regardless. But that's fair because the bus driver told you it may or may not get there, but you gotta buy the ticket if you want to get on the bus....and hope for the best.
No, I understand what you are saying. Yet in most anything else, contract law assumes an implied warranty for what the product or service is presented to do.