"These are quite OK. The data will be deleted after the scan. It will only be used where there is already suspicion, and never ever to check people guilty of walking while black.
So it's quite safe and OK. There's no need to protest."
Very shortly after they have become widespread: "We've just made these teeny, tiny changes in the conditions of use. And we have really good reasons for needing this."
A few months later: "Ah, just one slight extra adjustment to when we use them. Oh, and just in case of terrorism, we really do need to retain all the data".
Etcetera. Etcetera. Etcetera.
Once they have these, there is no limit on what they will want to do with them and will demand the right to do.