So, going by my layman's memory of what I learned many years ago in school:
The U.S. Supreme Court, or any part of the judicial branch, does not create laws. It only interprets laws which are created by the legislative branch, and sometimes decides whether or not they violate the Constitution. Therefore, the Supreme Court rulings in 1961 or 1968, for example, did not change the law, they clarified the intent of the law. Based on this, it appears to me that the Terry stop (which had been common practice for a long time in 1963) was not illegal.
Here’s the thing. Even if you want to argue that a “Terry stop” was constitutional before the Supreme Court majority invented it, what the Dallas police did to Oswald did not qualify.
“We merely hold today that,
where a police officer observes unusual conduct which leads him reasonably to conclude in light of his experience that criminal activity may be afoot and that the persons with whom he is dealing may be armed and presently dangerous, where, in the course of investigating this behavior, he identifies himself as a policeman and
makes reasonable inquiries, and where nothing in the initial stages of the encounter serves to dispel his reasonable fear for his own or others' safety, he is entitled for the protection of himself and others in the area to conduct a carefully limited search of the outer clothing of such persons in an attempt to discover weapons which might be used to assault him.” — Terry v. Ohio, 392 U.S. 1 (1968), (emphasis mine).
No police officer observed Oswald or the other two men in the theater engaging in any unusual conduct prior to being detained and searched, nor did any police officer make reasonable inquiries first.
And in any case, probable cause was absolutely necessary to make an arrest for murder.
