I'm sorry, I think I still haven't quite addressed what you said. There are basically two issues here: the first is that researching where memes come from is soft news and good journalists should be trying to find something more important to report on. The second is that they've taken a story about Trump and then wandered off on a tangent instead of sticking with Trump.
I hoping I'm characterizing your complaint accurately. For the first point: this is an instance where soft news and hard news overlap. Calling Trump on his bullshit has turned into it's own little industry at this point, and everything that he says gets researched. I'm not saying this is exceptional, it's possible that if a different president posted a meme then maybe that would get researched too. But when this president posts a meme, you can be damn sure that it's going to get examined. Examining what the president says, thoroughly, falls under the "important hard news" category.
For the second point: maybe. Maybe they did wander off on a tangent, but the story kind of created itself. They called this guy and didn't get in touch with him right away, left a message on his voicemail or something, and he apparently freaked out. He wrote the apology on his own initiative, edited his old comments on reddit to make himself seem a little less racist, and then called them begging them not to reveal his identity. What are they supposed to do with that? Just ignore it? What happens when people start asking about what happened to this guy? What happens when someone else starts looking into this meme that the president tweeted?
Setting those questions aside, it's probably worth pointing out that the president is apparently listening to outspoken racists. You do acknowledge this above, but just saying this without identifying who you're talking about is pretty useless. Worse than that, it's inviting denial.