Why would Brad go to jail for refusing to rat Donnie out?

You don't have to talk to the police, or testify at your trial. But if they have enough evidence against you, then they can still arrest, put on trial, and convict you, regardless if you talk.

They had more than enough evidence to put Brad away, but offered him a plea deal if he ratted out everyone else, including Donnie. They didn't do, so he stood trial, was convicted and went to prison.

We will see how a similar scenario plays out for Donnie the defeated former guy from Mar-a-Lardo.

I didn't get that either. I say its a plot hole

Brad got a jail sentence for his own crime. As Sandman81 rightly says, he refused a plea deal with the prosecutor in which he would have received a reduced sentence and no jail time for ratting out his business partners.