Okay, let me give you an analogy: Let's say I created this really fun game on my computer, and posted videos of it to YouTube showing off how great it was. Everyone sees it and wants to play it, but for whatever reason I decide to be a dick and not share it with anyone. (If I actually made something like that, I wouldn't really refuse to share it of course; this is only hypothetical.) Now let's say I upload it to my Google Drive account so I can download it on my other computer, but I accidentally upload it to a folder I made public.
I quickly notice my mistake and move it out of there, but before I'm able to, some lucky bastard with a fast Internet connection manages to download it. Said lucky bastard decides to do a good deed and re-upload it somewhere, and everyone who saw my videos finally gets to play it.
But I, being the asshole I am in this hypothetical universe, enjoyed seeing people complain about not being able to play it, so I decide to hire a bunch of lawyers and sue everyone I can find who decides to keep playing it. I think to myself: good thing I thought of the possibility of a leak and included a "license agreement" in the files that said, essentially, "nobody may use this software except for me!" Now my lawyers can say, "You are playing this game, but you do not have a license permitting you to do so!"
If, from a legal standpoint, the people I'm suing are actually in the wrong for that reason, that is just messed up.
I'm aware Nintendo isn't just trying to keep people from having fun (seriously, of all companies?) but still, they aren't harmed in any way by some random person on the Internet playing around with a leaked SDK without any desire to take away sales from anyone, so they shouldn't be able to abuse copyright law in that manner. (Though I am aware that "should" isn't necessarily the case in our legal system, especially when big companies are involved.)