You only play for what you use. This rewards anyone who mainly stays on wifi. My bill is roughly ~$35 a month with them while everyone I know has bills of 70-120. If you sign up for 2GB but use 1.1GB you only pay $11.
> A T-mobile family plan with unlimited data (throttled after 2GB) comes out to ~$25/month.
This may be a legacy plan; the only thing they seem to currently offer is $40/line/month for 4 lines ($70/month for one), but throttled at 50GB/month (not clear if this is by line or account), not 2GB/month.
It's a little cheaper than Fi list price at the level Bill Protection (essentially, unlimited with 15GB/line throttling threshold) kicks in ($160/4 lines with TMo vs $205 for Fi). But Fi does refund if you use less than 14 GB total, which may reduce the effective price depending on usage pattern.
User Agent sniffing, traffic analysis, port sniffing and new phones might even tell them. The carriers are already doing a lot of load balancing among all the devices attached to their network. It wouldn’t be too hard to detect.
Years ago AT&T sent me text right when I connected to my hotspot. I can’t see the tethering data use on AT&Ts dashboard but I can see usage per line.