It is absolutely illegal in Nevada. Not because of the width. But, because it is unrestrained from forward movement. Even if you wrapped ratchet straps around it, teh minute you hit the brakes hard that load is snapping them and continuing along it’s merry way. I’ve been to accidents, where the bar was 300-foot down the freeway, stuck in a Toyota.
They first appeared in the Vegas area about 50-years ago. A commercial steel company built them for their crew trucks, so they could haul bar to small jobs, or a bit more bar to a large job, if they didn’t need a whole truck load.
Theirs were engineered, and had stops, tie downs and a twenty foot long base. Actually worked really well. Since then every independent guy who ties bar, has built a poorly thought out imitation.