But it does...this is how cellular devices have updated for years. Only somewhat recently they started pushing updates to wifi only and that's out of selfishness of not bogging the network down with terabytes of system updates when a new one is rolled out to millions of devices. They can then put the burden on you now that they tend to be fairly sizeable.
In a closed system that has a single means of connectivity where else should it get data? The hotspot isnt smart enough to distinguish the data type, you'll likely dont want it to be either unless you like hotspots that catalog everything you do and mine that data for whomever. You wouldnt be happy if you had to hook to your phone to update either and the hotspot cant hook to home router so you're just stuck. If you're using a paid hotspot to offset phone data usage the investment us more than likely in the wrong place. I suggest looking at an upgraded phone plan. I've seen few instances where it makes sense.
Grand scheme of things Ford/ATT are just letting you sample this tech, they fully expect you to pay for it and do not care. After all it is an $80k truck.