It's reportedly for his own safety.
Things aren't look good for Tekashi 6ix9ine these days. After an incident at a Texas mall earlier this year where he allegedly choked out a teen, a warrant was issued for his arrest. The rapper's been on tour in Europe for the past month for his "World Domination" tour. Unfortunately by the time he landed, police were already waiting for him. Since then, the rapper's been denied bail and will remain in jail until his hearing on July 20th. The rapper's currently in Rikers Island and he's reportedly been transferred to the Contagious Disease Unit for his own safety.
According to TMZ, Tekashi 6ix9ine's status as a celebrity has forced authorities to transfer him to the Contagious Disease Unit. Law enforcement sources said that he'll be inside the 140-cell portion of the prison which is typically meant for inmates who are the battling illnesses. It's also used for celebrities who can't be in general population, for their own safety and 6ix9ine apparently falls in that category.
Tekashi 6ix9ine is reportedly being held in isolation. The rapper will have absolutely no contact with any other inmates even during his one hour of recreation. The rapper will only be able to have contact with the correctional officers.
At the time of his arrest, his team said that they were aware of the warrant but didn't feel like the NYPD had any business involving themselves with it because it was issued in Texas. However, they plan on dealing with the situation and getting 6ix9ine freed.