OK.
It’s important to understand the different levels of suspicion that can exist and what kinds of citizen/police interaction they justify.
Even with no suspicion, police are permitted to approach anyone and ask them questions. This is known as a “consensual encounter.” It doesn’t implicate the Fourth Amendment at all, because it’s completely voluntary. As long as the citizen is free to disregard the police inquiries and go about his business, no seizure has occurred.
If the police have “reasonable, articulable suspicion” of a felony, or of certain misdemeanors involving public safety, they can briefly detain (“seize”) a person to investigate and either confirm or dispel their suspicion. Reasonable, articulable suspicion exists when there are specific, articulable facts that give rise to a belief that a crime might be in progress.
To arrest a person, the police must have “probable cause” to believe a felony was committed. Probable cause exists when the totality of facts known to the police supports the belief that a crime has likely occurred.
And of course to ultimately convict someone of a crime, there must be proof beyond a reasonable doubt as to each and every element of the crime.
The police exceeded their authority because the report they were given described a misdemeanor, not a felony, and when they arrived they did not see that misdemeanor in progress. So the rule is that a completed misdemeanor that doesn’t involve a risk to public safety cannot serve as the basis for reasonable suspicion.
For this reason, police were certainly free to ask the pair questions…as long as the pair were free to disregard those questions and go about their business. But the moment the police seized the young lady, they implicated the Fourth Amendment’s guarantee against “unreasonable searches and seizures.” The courts have decided that a seizure for a completed misdemeanor based only on reasonable, articulable suspicion is not reasonable.