Because this weapon is completely overpowered and goes against the style and feeling of the game. When you can scan a whole area in seconds it makes it very difficult for people to implement any sort of sneaking and hiding behaviour. As an L85 user myself I agree with removing thermal because I feel I have to use. If I don't scan and see the enemy sniper first they'll sure as hell see me with their own thermal scope well before anyone would have a chance to spot each other without it.