I challenge you to find a Chrome extension in the Chrome web store that lets you download Youtube videos. Does Google allow, say, Amazon app store app in the Play store?
Apple doesn't have a search engine of it's own so I cannot point you to search results showing how to use other software instead of Apple's but do you really think they would censor the results of a search engine? The comparison is irrelevant.
What about that time when Google banned Disconnect from the Play store? [1] Meanwhile Apple actually built a way to create content filters into the OS and actively promotes them in the App Store.
I'm not saying that Apple is without fault, certainly not, but neither of the big companies are a shining beacon of enlightenment, and every one of them operates out of self-interest.
[1] http://www.theinquirer.net/inquirer/news/2411562/google-slap...