Think about it this way - the service provider is always using a managed router - it could be on-premise - and/or it could be the other end of the circuit back at the DC where all their customer's plugin (yep, you all plug into a router they manage).
But either way, they have to have managed routers (often many, and often L3 switches as well) somewhere to deliver Internet access.
Their whole business is about selling you access to the Internet, and to that end, they will make sure that what you are buying will enable that business model.