When talking with people they sometimes say 'I did nothing wrong, this is just for the bad guys'. But what people fail to see is that 'bad guys' is a pretty fluid concept and it depends on the nature of the current government and who decides which people are on the lists.
Looking at the current British government it isn't even that hard to imagine the wrong hands that this technology could get into and cause damage.
Who maintains the databases with 'wanted faces'? Imagining a some corrupt corporation that just puts their enemies on these lists, think union leaders, critical journalists, etc, again, isn't that hard.
Never mind that this tech, being tech, isn't perfect and there are probably false positives that lead to very annoying situations.