Why?
Prison grapevine. He's still in the county jail. He's been sentenced already, and can only remain in a jail for a specific period of time before having to be transferred to a State prison. His reputation will go with him. Criminals and inmates have long memories. I spent 25 years in uniform in NY State's prisons, and saw street grudges from years before, played out behind bars.
Because you are showing that you are weak and vulnerable.
Prey for the predators.
Until he proves that he is willing to fight, he will have to trade off sexual favours for protection.
Showing weakness is the worst thing to do when going to prison.