I understand what you mean, the problem however is that by wearing clearicals to a political rally it gives the impression that the Church itself endorses the politician, and not the individual Priests, or even that they are just neutral spectators. This would be just as true if a couple Priests sat behind the podium at an Obama rally. The military operates the same way. A military member cannot wear a uniform to a political rally because it gives the impression that the military, not the individual, is endorsing something. This is also why doctors are asked to wear their white coats to political rallies- so everyone knows that “all doctors” support whatever medical law is being proposed.
The Bishop is merely asking his Priests to avoid havin such conclusions drawn.