Why is selling Adblue emulator not illegal?

GasGas:
The get-out for their sale is that they can legitimately be fitted to trucks that will no longer be used on UK roads.

I know DAF has a policy that their dealers can only remove the emissions equipment from ‘export’ trucks after they have been sold.

Fleets which are caught using them are in for a world of pain…

transportoperator.co.uk/2017/10/ … -sabotage/

This interesting I did not see before. Still I do not understand why do they not just make selling and fitting device illegal then it much harder to get in first place