Accuracy
There is a small difference between training it on a set of mugshots of predominantly black suspects and then letting it loose on cctv of the Notting Hill carnival - and training it on high quality full-face ID cards and then comparing them to well lit full-face scanning machines.
If they are allowed to take samples of your piss and watch to make sure you have the right set of dangly bits for your sport - I don't think civil liberties are really a concern