re: Why are data providers so dead set against tethering?

For the same reason these cable companies are so opposed to net neutrality while many ISPs fully support it. They sell you a service with the assumption that you will never fully use it. Whether it's bandwidth or mobile data, until recently, outside of a few power users, it was pretty far-fetched to think anyone would actually take advantage of it.

They want to sell you "faster" services for higher prices while doing the minimal amount of infrastructure improvement necessary to facilitate it adequately. If people are tethering, they're now taking their data consumption to a completely new level where perpetual HD streaming and giant downloads are commonplace. Networks just aren't equipped to handle it. The same is happening with big cable companies who charge through the roof for 30Mbps and piece of shite ISPs selling "broadband up to 10Mbps*" to people with the asterisk clarifying that it's DSL/ADSL and will never come close to that speed under normal use.

In short, they want you to pay them for a blow job while they only intend MAYBE to touch it for a second or two.