Yes, it is. Your son was not committing a major crime, but he was urinating in public. Thus, the police had the right to place him in custody (or arrest him) and give him a citation for public urination. Any time the police can place you in custody, they are allowed to conduct a search of your person to look for weapons. The courts have upheld this as legal because of the fact that arrest situations are so dangerous to the police, that the courts have said that it is reasonable for the police to search for and seize weapons for their own safety. And this is exactly what happened in your son's case. The police seized your's son's knife during his "arrest" in order to protect their safety, and since the weapon was not evidence in the crime of urination itself, it will be returned.
I hope this answers your question.
Yes, they did not arrest him. But he was placed in custody meaning that he was not free to leave. And even in these circumstances, the police are allowed to search for weapons for their own safety.
DISCLAIMER: Answers from Experts on JustAnswer are not substitutes for the advice of an attorney. JustAnswer is a public forum and questions and responses are not private or confidential or protected by the attorney-client privilege. The Expert above is not your attorney, and the response above is not legal advice. You should not read this response to propose specific action or address specific circumstances, but only to give you a sense of general principles of law that might affect the situation you describe. Application of these general principles to particular circumstances must be done by a lawyer who has spoken with you in confidence, learned all relevant information, and explored various options. Before acting on these general principles, you should hire a lawyer licensed to practice law in the jurisdiction to which your question pertains.
The responses above are from individual Experts, not JustAnswer. The site and services are provided “as is”. To view the verified credential of an Expert, click on the “Verified” symbol in the Expert’s profile. This site is not for emergency questions which should be directed immediately by telephone or in-person to qualified professionals. Please carefully read the Terms of Service (last updated February 8, 2012).