There are 2 ways to see that, and why it's correct.
1. If you check the 2nd rule under the NY one. That is the Arena (Cube and Coliseum) rule, it specifically mentions not using it to avoid RP. Same goes for any "protected" system. No RP in Arena, then do not use it to avoid RP. No RP/PvPs in NY, then do not use it to avoid RP.
The rules are short and mentions the basic ideas, but it is also for people with common sense, since "reading it word by word" will always bring out openings for exploits, on any rules list (even real life law, which is like 1000 pages), and comes with the players who "go around the rules", focusing on a "word" that mentions half of the story. But that doesn't mean you can do the other half of the story and not get punished when caught.
There are several openings in the rules which can, and did were abused in the past. That is where the server police comes.
2. Anyway, the NY rule says " Experienced players may (1) transit through this system or (2) assist new players here otherwise (3) they should avoid New York."
Going to help a new player there is ok, case Nr 2. Going to help a new player after placing a bounty on a cop and being chased by cops (so, adding a RP situation in the story), is NOT case 2 (it doesn't mention the part of being in a RP case, so if it's not mentioned as "exception", then it is not an exception), neither case 1. So if it's neither case 1 or 2, then it's case 3: "otherwise, they should avoid NY".
Soo... Wether you pick to strictly read what the rules are saying (and see that RP+ helping a new player != helping a new player), or if you use common sense to see it from around the rules (no RP in system XY, then do not go to system XY while in a RP case), you can see, IF both Gunny and Pui are correct, then Pui did as wrong by going to NY as much as Gunny killing him in NY. But none of them is a bannable offense, at least, not in the first times. And anwyay, not a reason to feel "abused"... it'd be an abuse if Pui was a newbie.
And t here is the info that players above 40 are not protected there, so it would end up that killing a lvl 59 player in NY is ok, but it's not visible in the rules (yet).
PS: "!=" means "not equal", in this case, "not the same as".