How does non-custodial and custodial interrogations differ for the purpose of miranda warnings?

If a question is asked of a suspect in a noncustodial setting (i.e. free to leave) then Miranda is not required. Miranda only applies to custodial interrogations/questioning.

You only have to be read your Miranda warnings/rights if you are in custody by the police. If not in custody the law does not require your rights to be read because at anytime if you are not in police custody you can leave the interrogation/questioning. Now when you are in custody your rights have to be read and upheld, usually when a suspect speaks while in custody and Miranda rights are not read to them they don't know that they do not have to speak without proper counsel/defense. The purpose of Miranda rights is to give the suspect an opportunity to utilize our Justice system and be given a "fair" chance to defend themselves or not to convict themselves unknowingly. It's sad to say that there are a few bad apples in the bunch that use their authority in some cases to get confessions out of people through manipulation while not in custody and then after they get the suspect to confess or give them the info they want to hear they begin to read Miranda rights and book the suspect. Manipulation is not illegal to gain a confession but I feel the authorities should not make a suspect that is only being questioned and not in custody feel that they are in custody and intimidated to leave.