Yeah, I think you are misunderstanding. According to the law a Terry Stop isn't a detention but a safety frisk as you said. It isn't a detention at all, just officer safety.
I don't believe this is correct.
First, "Terry Stop" isn't defined in the law; it's established by case law. Primarily Terry v. Ohio, but with clarifications from subsequent cases. If you read Terry v. Ohio, it's pretty clear that the original "Terry Stop" most definitely was a detention. Terry was not free to leave.
To really explain why I think your interpretation doesn't make any sense requires a little background. Please bear with me for a moment
Police interactions with citizens fall into one of exactly three categories: consensual, detention and arrest. This is important, because when we understand these categories, it becomes pretty clear which one a frisk must fall into.
During a consensual encounter the officer does not have reasonable articulable suspicion that the citizen has committed a crime and therefore has no power to compel the citizen to do anything. The citizen doesn't have to identify himself, doesn't have to answer any questions, and certainly doesn't have to submit to ANY kind of search, not even a frisk for weapons.
To escalate the encounter to a detention requires that the officer have a reasonable articulable suspicion that the citizen has committed, is committing or is about to commit a crime. This gives the officer some power to compel the suspect. As the name "detention" implies, the officer can prevent the suspect from leaving for a reasonable amount of time, while the officer investigates and attempts to confirm or dispel his suspicions. Under Utah law (77-7-15), the officer can also compel the suspect to identify himself. The officer can't compel the suspect to answer questions beyond identifying himself, but usually people will answer questions in an effort to dispel the officer's concerns and end the detention in a positive manner.
To escalate the encounter to an arrest requires that the officer have probable cause, i.e. evidence, that the citizen has committed or is committing a crime. This gives the officer much greater power. He can conduct a complete search of the suspect and immediate possessions. He can transport the suspect and hold him for a much greater period of time (a day or two). And, finally, he can charge the suspect with the crime he has evidence of.
So, into which of these situations does a Terry Stop fall? Clearly, not the "consensual encounter". If it did, then police could stop and frisk random people with no basis at all. No frisk could ever be unlawful. Also, a frisk clearly requires at least a brief detention -- otherwise the citizen can simply walk away from the frisk. But a consensual encounter gives the officer NO power to detain.
Even more clearly, Terry Stops don't fall into the "arrest" situation. In that situation there's no need for a simple frisk; a full-blown search is allowed.
Therefore, frisks can only happen as part of a detention.
The way I see it is a Terry Stop (really a Terry Stop-and-Frisk) is just that, a frisk for weapons, nothing more.
Yes, but there must be something to justify it. As I pointed out above, police officers can't frisk random people on the street, not even to ensure officer safety.
An example is an officer has a mere suspicion that someone is casing Walgreens at 1am in the morning. That officer can go over and talk to the person and perform a Terry Stop (stop-and-frisk) during asking routine questions. But if he is nailing you for ten minutes with question after question "searching" and "grabbing" for something to nail you with, then you'd probably have a reasonable expectation that you are under arrest a CLaP states.
In this scenario, the officer has a reasonable, articulable suspicion that the suspect is casing Walgreens -- that gives him the authority to detain the suspect and investigate. In the context of a detention, the officer can conduct a frisk to assure his safety. If the officer goes beyond a frisk, to a real search, then the suspect should assume he is under arrest, and if it turns out the officer didn't have probable cause then the suspect has grounds to sue for unlawful arrest.