From my reading since I've adopted my rabbits, they can eat straw but it isn't good nutritionally like hay is. As long as they're getting good hay as well, I expect they'd prefer that and I wouldn't worry too much if they were snacking on a bit, but I'd be wanting to see their good food eaten. You could try just putting the straw on the bottom and hay on top so when they snack they are getting "the good stuff".