NPCs don't vanish at 1% health, invalidating the effort destroying them and the influence gain.
Why should players be able to do that,
Because the consequences of trying to legislate it are worse.
when that gain is what drives PP? Open needs less holes to make that outcome be 'valid'.
It is de-facto "valid", even if you don't like it (or me either, but that's beside the point.) If it bothers you that much, then adjust your playstyle. Don't play in Open? Maybe add a few severe cases (I can't imagine this "1% quit" playstyle is really popular) to your blocklist, etc., where your supervenient ruleset permits it?