Right, and I think that part of Flock should be fixed. But I don't think in general it is a bad idea. Do these same people also think it is bad that the police can get a warrant to read your text messages?
My real argument would probably be that this system is probably not unconstitutional but is very problematic and easily abused. Flock gets the hate that it does because it can be abused, and has been abused, and will in all likelihood continue to be abused.
All kinds of ideas - especially involving having a very powerful government - would be great in theory if we could have "the necessary safeguards", but the necessary safeguards do not exist and I am not sure if they can be made to exist, at least in a practical realistic way.