Sure, you can arrest any priest... But then what? You can't really do jack !@#$ with them once you have them in your clutches. You just stare at them awhile, then they go home and preach again. And if they RTO a region or two? Nope, you can't even ban those suckers. Cheerio, pip pip, have a good time destroying our infrastructure, see you again on Monday!
IMO, you should be able to ban -any- noble that ends up in your prison, just at a hit to honour/prestige if they aren't already an infiltrator or rebel commander.
