A company with a 475B market cap doesn't just do things to be "elitest and a d*ck". I often see these arguments that Apple does things just to be mean or evil but frankly, I think these are emotional responses. There is a more objective reasoning behind why they do the things they do.
From the beginning, Apple has been about presenting a certain "consistency of quality". They want people to equate an Apple product with a product that is free from worries. I get it. You may not agree with it, but I get it. This will naturally piss off 5% of the hard core users who want ultimate flexibility from their platform. But it will also make 95% of the people feel all snugly and warm that they don't need to worry about things like porn or viruses appearing on their device.
Again, I'm not arguing whether 500px's app measures up to "objectional material" or not. What I am arguing is that the reason why Apple removed the app is not because they're trying to be mean but because they have discovered that an app (whether it's had 1,000,00 downloads or 100 downloads) violates the "consistency of quality" that they are trying to present.
Frankly, the 8 year old version of me would have had no difficulty visiting the desktop version of the website and disabling safemode.