I am not a Constitutional expert, but since this is required for passports, drivers' licences, military ID's, and in some states work and college ID's, it seems to me that complaining that this is unconstitutional is a bit late.
I don't want to curtail people's rights, but it seems to me that when you leave the country and attend a suspect organization's rally, coming back is going to be inspected a little more closely, especially during a time of war. No one was asking for their fingerprints until they left the country and attended this convention. Free speech doesn't exend to other countries.
If these people presented NO acceptable ID, they could have been presented with the option of giving fingerprints and photos to be checked BEFORE entry, or to simply turn around and go back.
Through checking of their ID does NOT entail taking photos and fingerprints AFTER they have proved their right to entry, no matter where they have been.