They didn't ban SEA, they are simply enforcing policy. That policy hasnt been updated since 2016. Either willingly or by the hand of Valve after the HellDivers 2 debacle with Sony.

They shouldn't be selling the game to regions that they don't support. It is SE fault for taking this long to patch the holes.

The most likely reason is local laws and taxes, not worth the hassle or cost.
It is what it is, but people dont read the policy when making accounts.