There are one or two stories a month on gaming sites about Apple pulling/rejecting games for reasons that either don't resemble reality or are just bizarre. For example they rejected a WW2 game because the Japanese faction in it was based on the Japanese in WW2 and doing that broke their guidelines - "violent" games aren't allowed to have enemy factions based on real races, cultures, governments, corporations or any other "real entity". They also famously don't pull lots of games that they should be pulling - like direct rip offs of existing titles that are dressed up to look like the real one.
As for the specific allegation the BBC have highlighted (probably because they know people will froth at the mouth over the mere suggestion, even if it's baseless) there are bound to be photos of models that look quite young on it and lots of people treat anything with nudity as porn.
I can imagine someone downloading it to look at the wildlife or landscape photography and not being thrilled to find art nudes there too, though I was under the impression it respected your account filtering setting and defaulted to no nudity like Flickr does.
If they were serious about this they would pull Twitter, Tumblr, Instragram and everything else like that because they have much less effective filtering systems if they even have any at all.