I understand the arrest rules. The point is both gangs usually approach cautiously using terrain for concealment and so a judge usually has to expose himself to fire in order to make an arrest.
Whereas a pimp could approach a judge, inflict damage with incinerating finger and not be automatically liable for arrest. In fact a could of psi muties could conceivably take out a street judge, not be seen by the rest of the Judges and get away scot free.
He's still liable for arrest, just not for being shot out of hand.
Well, that kind of makes sense from a story perspective as well. If you've shot at a judge, arrests go out of the window and he can slot you with a lawgiver.
If he's been burned by "spontaneous fire", he necessarily can't
prove it was you, so he (or his partner if he's down) still has to try and arrest you. It's the same as heroes - you still have to
try and arrest them if they haven't opened fire yet, even knowing they'll pass the WTF check automatically.
By comparison, Psi-Judges know it's you (see Anderson in the
Dredd film), so they can probably get away with using psi talents to bring you down, but an automatically registered and theoretically unprovoked lawgiver discharge is probably pushing it too far.
Essentially, as you say, it essentially hands 'first shot' to a non-judge gang because the judges can't use the full range of their lawgivers until bullets start flying, and they have to risk a shot to make an arrest.