Again, not reading his rights is not the same as not letting him
exercise his rights. It only means he will not receive a reminder of those rights, nor a direct question as to whether he understands them.
Miranda did not create the right to remain silent, nor the right to an attorney, or any of the other things the Miranda advisory discusses. Those always existed and people could always exercise them. The basis of Miranda was that the court determined that the police could not simply assume that a person understood their rights against self-incrimination. After that case, a Miranda advisory was required in order to use a person's statements against them.
The reason they aren't reading him his rights has nothing to do with him, at all. It has to do with not talking him into clamming up about
other people he or his brother might have been involved in. A person's rights exist to protect that person, not to protect anyone else, so there is no reason he needs to be read those rights in order to gain information for that purpose, not to mention that it would be intelligence information, not for introduction in court at all.
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My suspicion is that they feel they have more than enough hard evidence to convict him without needing any statements from him directly, so it's all about info gathering around other possible threats.
This. There is nothing the least bit sinister or unusual about someone not being read Miranda. It's done every day, because crimes occur all the time where no statement from the subject is needed. Driving with a suspended license is a perfect example. You observe a violation, stop the person, in the process of writing the ticket discover they don't have a valid license, make the arrest, process it, and off to court it goes. The entire case is based on what was observed. Even if you ask the driver something like "did you know your license was suspended?" it won't be admitted in court because it's both unnecessary and irrelevant, so there's no requirement for Miranda.