That would be because they are not contracted with Apple. If they're piggybacking off another carrier but serializing their own services, in the US, Apple will deny you access to hotspots because Apple does not recognize nor do they authorize the carrier for use with their device. The iPhone 5 unlocked brought to MetroPCS did not warrant users hotspoting despite them buying or paying for a hotspot plan, nor does Apple intend to give MetroPCS iPhone users the ability to hotspot with their devices.
In a capitalist society, if you're not getting paid for your brand, there's no need to allow others to do as they please with your product