"This must vary by state not "U.S. government rules" because I have a Ford F150 that is by most definitions a "truck" but it is legally registered as, and taxed as, a passenger vehicle ("suburban") not as a "truck" and as such it is legally allowed on parkways, which in my state at least, are highways that specifically outlaw trucks and any commercial vehicles. I know of no one in my state who's SUV was legally considered a "truck" it is purely a social term for the vehicles."
National regulations involve the manufacture of the vehicle and safety regulations for selling them, and taxation on import. Registration, sales/use taxes and fees are always handled at the state level so that definition will vary by state. By U.S. government rules, which cover the safety tests they must pass, emissions and engine standards, and design rules, SUVs are trucks. I wouldn't see a problem saying they are "cars" in the passenger vehicle sense, but I do think that saying they're not trucks is inaccurate because of the above.