At one point Wolverine, SFRC and a few other stores sold 5.45x39 ammunition, so there must have been a small demand for it, probably for a few people that had AR15 uppers chambered for it.
It isn't inconceivable that these same people have decided to install their parts on one of the new straight pull receiver sets available.
Anyone suggesting that the OP is trying to obtain ammunition for a prohibited AK74 simply has a vivid imagination.