I assume because it's "metered" (ie, billed by byte). The TODO suggests that this should be figured out rather than always enabled. You can go to "Settings >

I assume because it's "metered" (ie, billed by byte). The TODO suggests that this should be figured out rather than always enabled. You can go to "Settings > Data usage > ⋮ > Mobile Hotspots" and maybe that's related? I've not checked/experimented. But it's possible you can go there and disable the feature for that AP.