Notice the police cruiser cameras? No warrant needed for sound and video for them.
Is court testimony based on them then, legal? It's possible that such recordings might be used (until some lawyer challenges it, anyway). Police powers now encompass so many things now, some directly in conflict with the constitution (if a cop mentions "drugs", for instance, property can be siezed, and sold at auction without restitution to the owner, even if the charges never go to court. DUI checkpoints allow basic searches without probable cause... etc., etc) and precedent. An officer can 'run' a license plate (public) without any overt reason; asking for identification, license, insurance, registration, etc. requires probable cause. So - is a face a 'license plate', or 'ID'? Can the police legally use face-recognition software to trawl for 'perps'?
Since, as many love to quote, driving is a privelege, not a right, license plates might be spot-checkable - I suspect that since "Life, Liberty, and the Pursuit of Happiness" is a holy canon here in the 'States, 'face checks' don't fall into the same category.
And if they are tested legally, and are considered legal, watch hoods and masks become commonplace fashion statements. Not everyone is comfortable with the police watching everything you do, with the only respite being to hide on private property.