In a mesh network each node becomes its own ISP basically.
They don't. If a mesh node wants to access the internet, it still needs to access via a centralized service provider. How else is the mesh network going to connect to the internet? There's no other way to access it, not even on a physical level.
It's a shared gateway that does not charge a user fee or identify users identity. The mesh itself is decentralized and secure. No user is connected to a centralized ISP.