Yes, if they read your Miranda rights, you are under arrest. However, they still don’t have to tell you why until you are arraigned.
“However, they still don’t have to tell you why until you are arraigned.”
I absolutely agree. But that was not the debate. The debate was the difference between detainment and arrest. They cannot “detain” you for three days. They can only temporarily detain you but have to make a decision at the scene of the interaction whether to arrest you for further legal action or cut you loose. And some states actually have a time limit they have to make up their minds which.
I think where we may be having a misunderstanding is with the definition of detainment. There are technically two types. One being temporary being detained from your right to travel freely pending local investigation, And being detained 72 hours in jail and held over for arraignment after arrest.
They are legally two totally different instances of the same word “detainment”.