The fourth Amendment to the Constitution does an effective job of preventing this country from turning into a literal police state, by virtue of the fact that the people have the right to be secure in their own persons. One might think, that the right to be secure in their own person would also apply within the public arena, but when the policing arm of the state are allowed to stop-and-frisk people at will, then the people are most definitely not secure in their person.
The famous Supreme Court decision in regards to Terry v. Ohio, was decided in 1968, of which the case involved whether or not a police officer was entitled to make a determination based upon their experience as to whether a given situation warranted the police officer's discretion to stop and search citizens, simply based on the fact that those citizens looked suspicious to the police officer. The Supreme Court decision, decided that indeed, police officers were allowed to do just that, and therefore it so follows, that stop-and-frisk, is judicially considered legal, just as long as such stopping and frisking is not done via simplistic racial profiling or used as a policing tool of harassment.
Of course, the thing about the law, and the interpretations of the law, is that certain laws in many respects are not fixed, and that such laws may, in fact, evolve over time, for better or for worse. When any government, makes it a point, that the policing arm of that state can stop-and-frisk citizens, at will, then the people as a whole, have lost something of significance, to get, at best, in theory, a safer society. Of course, there are many people that will sacrifice certain freedoms for safety, as well as there being certain privileged people that will theoretically sacrifice some of their precious freedoms, for safety, when in actuality those privileged people know for a certainty, that they and their kind will never suffer the indignity of actually sacrificing much of anything. So then, what has really occurred in principle is that there are laws that are deliberately made and applied specifically against the disenfranchised, but only accidently applied against the franchised.
When the sovereignty of a given individual, depends upon whether they were born into favorable circumstances, or of a favored class, or are well positioned in society, whereas all others, must deal with laws that make them subject to the indignity as well as to the danger of being stopped-and-frisked at the will of the authorities then you clearly do not have a law that is equally applied to all but you have instead a law that is used as an excuse for the state to always have the upper hand and to be able to perpetually have their boot upon the people's neck.
In point of fact, the very structure of stop-and-frisk in its inception and implementation, is prejudicial against those that do not have the resources or the wherewithal to travel the public streets, other than typically by walking, and the authorities know that the structure of America is a structure that allows it impunity to treat those that appear to be of a non-protected class in a manner in which, those that do not demonstrate appropriate respect and appropriate deference to the policing arm of the state, will place themselves into the position of being, at best, inconvenienced, and at worse, subject to maltreatment and/or incarceration, if their obedience to those state authorities is not promptly demonstrated in action.