This is carrier specific.
In fact, between my spouse, my dad and I, we have 4 lines with Mint Mobile, and they do not prevent us from tethering or using our data as hotspots.
The 3 smartphone only plans are $15/month, unlimited talk/text, 4GB data.
the 4th plan is $20/month, 10GB data, and sits in an LTE hotspot, handling his modest home internet needs.
Where he lives (very rural), he'd have to pay thousands to get cable internet (or worse, DSL) run to his house, so this is a great alternative. But just a few years ago, this kind of thing was harder to find, harder to set up, more expensive, slower, with lower data caps. Competition helps, but the situation isn't perfect.