Another reason this could be briefs is that Justin most likely got the underwear for free as a promotion from SKIMS. Years ago, Cameron Dallas was a brand ambassador for D&G. He received briefs, and was caught wearing them while on vacation when his luggage was lost.
While Cameron has been photographed in briefs quite a few times, he is mainly wears boxer briefs. Most times he has been caught in briefs has been for photoshoots or brand partnerships.