Hmm... I s'pose I shoulda thought of that, janax/goldielax. But sometimes the raiders don't think this stuff through either, y'know? And others just want the challenge of the fight (NEW for example) and don't really care.
Then again, if one considered the Grämlins campaign against IRON part of the last global war (BiPolar/UJW2/whatever the hell you wanna call it), it could have just as easily been people being careful not to bring war back upon themselves by violating a no reentry clause.
Also, keep in mind, the war ended [i]before[/i] MHA cancelled the Härmlin Accords, so treaty-chaining does also play a small role. To attack Grems before the cancellation would be to invite the wrath of MHA, and it would snowball from there. On paper, anyway. (A few intangible factors might change that.)