They've been mandated in UK for 30 years - long before fuel efficiency would have been a consideration, but some of the new ones do look a lot more streamlined than 2 or 3 bars welded along the side of a trailer!
Come on USA, buck up and get trailer side impact bars mandated and start saving countless lives.
In UK the law change was retrospective, plus we are in the process of removing most of the exemptions - going back to vehicles built since 1960. Some large construction projects over here include contractual terms for vehicle safety that exceed legal requirements, and contractors have to retro-fit the additional equipment, send drivers on safety courses, and stick to designated (safer ...) routes when travelling to/from the site. Incentives like that help to improve the fleet