Not to mention Google tries hard to employ the same practices (you have to do some really roundabout stuff to install non-Play store apks).
Now tell me, how can I get an alternative app store, or an internet browser, that doesn't use crappy Webkit on iOS...
This article on how to install the Amazon App store and third party APKs pretty much sums up the situation. On the surface it seems like a great idea to allow third party apps, but in practice it actually leads to a degraded experience, viruses, and vulnerabilities, as evidenced by the editor’s note at the top of the article recommending against doing what the article is saying.