Most of the people whose devices and connections are being used as residential proxy exit nodes are not aware of it.

They likely charge per GB because these residential connections are slow and limited compared to datacenter connections (doesn't help that they're often located in third world countries), and are often used for aggressive scraping, so charging a fixed monthly price would not be viable.

Probably safe to assume that yours is. Especially if a teenager is using your wifi.

I can assure you they are not.