Is "not paying" effectively the same thing as unsubscribing?
I guess they could keep providing you the service and keep track of the debt you "owe" them. Once it becomes high enough they would find ways to claim the money.
Indeed. I tried once with a particularly shady company that required phone calls to unsubscribe. The service after 1 month unpaid, but the money was still owed and ultimately they sold the "debt" to one of those companies that will try to scare you into paying via threatening emails.