Maybe DM hasn't had access to scissors to cut his hair. I wonder how often the barber does the rounds and I wonder if he/she starts in the main population before getting (if at all ) to solitary.
Maybe being part native he likes the style. I don't think anyone would be ridiculing him if he was living on a First Nations reserve, so I wonder why the discrimination against him for choosing to wear hair tidily tied back.
The other possibility is that many people in jail grow their hair, there are no women in there, no need to stand on ceremony. I think that is quite possibly a reason for the longer hair. Tying it back would be to not appear anything other than presentable for court. JMO
I believe many people in jail do get their hair cut prior to trial, I don't believe that to be an anomaly that would solely apply to DM. JMO MOO