Monopoly or not, they still want your money. In many cars, you can subscribe to satellite radio service. There is only one company that offers the service. If you sign up, at the end of the agreement they raise the rates. Call and say the words, "cancel service" and a customer retention group comes into play and can offer you the same price as you had previously. In many places, cable is the only Internet option. Same as above, they raise the rates, you call to cancel, the customer retention group pops into action and gives you the pricing you previously had.
Alarm monitoring is not a requirement to have. It offers peace of mind. So even if a company is the only option for service, people can still cancel.