Because it's very likely that anything that would be seen as a "bug" will be fixed in updates; whereas things that are the result of intentional (bad) design, like lack of information hierarchy, probably won't be fixed, because they won't be seen as incorrect. (Which is, nominally, what this tumblr would be for pointing out.)
I don't think I agree with your last sentence, which seems to be the issue. The site isn't called "The Terrible Design Decisions of iOS7", it called "Sloppy UI" which I think covers both bugs and UI. Further, sometimes it can be hard to differentiate between a bug and a design decision, so I think it's hard to ask them to just stick to one or the other. May as well point out everything you see that isn't up to standards, be those design or implementation.
To me, the Tumblr is pointing out an overall lack of attention to detail. Some of these obvious bugs should have never made it past QA/testing, but somehow they did - and that's sloppy UI, regardless of whether it was an intentional "do that" directive.
I understand what you're saying, and perhaps the name of the Tumblr should change to "Issues with iOS7", but that sounds pretty boring to me. Instead, we can all agree that sloppy/bad decision-making, sloppy QA practices, etcetera can be learned from, and that there is value in identifying (and avoiding) these mistakes both in the works of others and in our own work.
