Why not, Sony does it too? I mean at least a little lower, it's totally ridiculous especially since as urfe already said retail games are sold for less than the suggested price nearly everywhere which makes the DD versions actually more expensive than the retail ones.
Is Sony selling more games or something? I don't think anyone cares what they are doing unless they have something to show for it. Sony is aggressively pushing DD because they do want to cut retailers out of the equation and they do want to sell expensive storage solutions. Not because they want to give you better deals.
Nintendo started to do 20% off weeks for certain digital games (with one already over and the next to follow who knows when), hopefully the DD versions of retail games will be part of these bargains in the future.
I imagine they will. It's fine for them to discount every once in a while, they just aren't going to have these odd everyday prices that are below MSRP.
Really Nintendo's DD isn't positioned as something that can be good or bad in terms of sales. They aren't trying to transition sales which is the single biggest difference between them and literally everyone else, they are simply offering alternatives. As such they will sell the exact same amount as they would have if they never had it but perhaps some people are enticed by having game on them all the time or will buy some out of print titles later on. 5% for Mario is just 5% they get a slightly bigger cut on. 20% for Oni Training means there's a big difference in the way people play and would like to own certain games and this is completely expected.