You're asking two questions:
Why would North Korea attempt this, if it was indeed them?
North Korean society is not as isolated as it once was, in part thanks to smuggled IT, especially on the borders. North Korean population is starting to watch South Korean television and generally consume international news sources more and more, which means North Korean leadership is either finding themselves the ones at a certain informational disadvantage, not quite knowing if they are sitting on a powder keg of dissent, or they feel they know what's going on and want to reign public opinion in. Either way, showing they are still a player on the international stage, even with something so ridiculous as a "made you look" kind of stunt, probably does the job, sad as it is.
How would US investigators know that it is North Korea, if it was indeed them?
Not everything is being disclosed. It's possible that the investigators or those in charge of public relations on the western side are overplaying strength of evidence. In some way, if there is a desire to tie this to North Korean, then this is a perfect opportunity, regardless of whether it can proven or not. In that way, it makes sense from an international relations point of view. At the same time, there may well be counter-intelligence shedding light on this, that the western authorities don't feel they can disclose. What's peculiar about that, is that situations like this give governments good information without ability to act on it for fear of erasing an intelligence advantage somewhere else. This was the case with the Rosenbergs. At the time of their conviction and execution, intelligence officials knew of their innocence, but the evidence for that was obtained through covert means and could not have been used, thus the tragic events were allowed to unfold. In the end, it's very hard for anyone not involved in this to parse out what's really going on.