DRLs are NOT required in the US.
From the Insurance Institute for Highway Safety: "No U.S. state mandates DRLs, but some require drivers to operate vehicles with lights on in bad weather."
Interestingly, General Motors petitioned the NHTSA to require DRLs, but they refused saying in 2009 "Based on the results of our study we were unable to find solid evidence of an overall safety benefit associated with daytime running lamps and are therefore denying the petition for rulemaking. The agency maintains its neutral position with respect to the safety benefits from the use of daytime running lamps."