it's a reminder of the old days where ISPs limited your connection to one device and required to pay multiple times if you had more than one device at home. people got wise to it and found that by using a router they could hide multiple devices and the practice eventually stopped.

it was based on the same argument that multiple devices would be used by multiple people and hence create more traffic.

i don't know if this was common in the US, but it certainly was in some countries in europe.