That's not true in theory or practice. They are supposed to issue a Miranda warning at the time of arrest or prior to interrogation; detainment is not arrest, asking questions is not interrogation. If they pull you over and ask how fast you were going, if you confess to speeding it is admissible. Even when Miranda violations do occur, if they're not on video they can lie about it. Police are allowed to lie about most things and they're trained to lie, it's probably their most-used tactic. Since they're human, once they step outside the truth it's tempting to remain there, especially if it benefits them. The word of police is generally accepted by the court as fact unless evidence directly contradicts them and that evidence is not suppressed.