Jets are indicative of some kind of asymmetry in the collimation mechanism. The optical jets in (e.g.) M87 and 3C 273 raise problems of their own, but the radio emission from the jets in NGC 6251 and NGC 315 may come from the beam itself rather than a "cocoon". There are two possibilities:
(a) The emission may come from particles accelerated by dissipation processes at the "walls" of the beam; or
(b) The electrons in the beam itself may not have been completely cooled by radiative or adiabatic losses during the out-flow from the nucleus, so that they are still able to contribute synchrotron emission.
Alternative (a) would predict limb-brightening and a magnetic field along the jet: but (b) would predict a magnetic field perpendicular to the jet.