The rules are very clear on this, it only needs a small amount of common sense to understand a situation and assess it.
The clear exception to the words "arrest first" is 'unless arresting is not an option', and after it you're given a couple of examples.
If you want the desired effect of your suggestion, then you have to suggest that cops should not be allowed to participate in Turf Zones at all, otherwise, the rules remain unchanged and the exceptions still apply. A cop entering a turf zone is almost always outnumbered, and most definitely taking fire, it makes no sense to allow cops to enter Turf Zones then tell them 'But you can only run around and try to arrest the 50 people shooting at you, oh and you can't shoot back.'
If you have an issue with a certain cop that's targeting you and constantly killing you, you can easily report that ingame without needing to suggest this.
This doesn't really seem like a genuine suggestion if you're gonna use the argument 'Cops kill every criminal', because if that was actually true then you can literally get them adminjailed without this suggestion.
To answer Tut's question, to me it seems like this suggestion isn't really because people think it's worse to get killed than arrested, it's more like... Criminals want cops to chase them instead of shoot back, that way cops become an easier target, but when cops have the ability to shoot back, they're not as easy a target because they could kill your teammate and arrest you. On the other hand, if the cop wasn't able to shoot back, the most likely thing to happen is that the cop will try to run straight to you and your teammate, making it way less risky and more easy to kill the cop.
TL:DR, it's easier to kill a cop when he's chasing you than it is when he's shooting you. That's why they want this rule added.