Jetfire was an Autobot who transformed into a military vehicle. Ditto Landmine, Comettor and arguably Brawn and Outback.

Disregarding Decepticon defectees and characters who don't appear in the film?? Umm... I dunno, maybe they only transformed into whatever NEST allowed them to scan? <shrug> Or maybe the Autobots have an ethical objection over transforming into vehicles of war and prefer peaceful alt modes (ya know, despite their "take no prisoners" policy ) -- yeah, I don't know.