I cant remember what I consented to on GedMatch now. Just looked on ancestry and it was just scientific stuff.Consent for "FBI and police to use" Ancestry doesn't currently allow police officials to access public DNA . However Gedmatch does with prior consent by the consumer, currently The rights to DNA in the UK at least is private to the owner and you can refuse to give consent. This is also why police collect evidence from discarded rubbish, to get around this issue
I believe however once a match is proven they can get a warrant to get further conclusive evidence.