It's because Webe uses TMGo's LTE infra, which was originally conceived to cover the areas not covered by TM's streamyx and UniFi. That's why you can see Webe's LTE at some ulu ulu places. Here in Sarawak, Bintulu, one of the largest towns has no LTE. But you can see LTE at some of the longhouses and smaller towns

Coz they dont want it to eat into their Streamyx/UniFi pie.

They should have a different quota for tethering. Say, 5GB maybe? Sometimes we need to tether in case the office or home fixed line is down

There is no way they're not gonna extend the promo judging by the length of queues they have