Update:  Got more info from support.

"The term hotspot gets used sometimes just because it is the most common internet device we offer but all internet only devices such as hotspots, USB modems, or other internet connected devices receive 15GBs of high speed internet per month. You do get 4G speeds for those first 15GBs but after that the speeds are lowered to a maximum of 600 Kbps.

Phones and tablets are the only exceptions to this because they aren't providing internet to other devices, possibly many other devices. Their internet is for use on that one device. If you use the hotspot option on those devices, they also only get the 15GBs of high speed data to use on the hotspot."