Because - assuming you don't reach your data cap every month - you might be sharing your leftover data with somebody else who's getting it "for free" as far as They (the carrier/provider) are concerned. They can't control who/what devices connect to your hotspot, so They assume every tethered connection is siphoning data to a non-customer entity, potentially disincentivising "the leech" from subscribing to their own data plan.

If They can steer "the leech" towards becoming a paying customer, then they can harvest (more) data & device activity from both users AND they have more active data plans (paying account holders) to boast about to their real customers - the shareholders - than they would have otherwise.

It's pretty simple really, you just have to think like an executive who's fiduciarily beholden to lining the pockets of shareholders (as opposed to a business owner trying to provide a useful & mutually beneficial service to their customers). The latter do exist in the corporate world, but they are few and far between when you're a publicly traded S-corp like most (maybe all?) of the major providers. It's just the banality of societally-accepted evil at its finest (and yes; utter bullshit)