Apple designed iOS in such a way that all its native applications open any kind of links accessed by users in iOS. When I say links, I don't mean only those for websites, because iOS works based on an internal system of links that allow developers to make the connection between applications and their functions. Basically, when you access a link in an application, you will open that link either with the application's internal browser or with Safari, but if you access an email link, you can only use the Mail application, and if you want to watch a video clip on YouTube you have no other alternative than the native iOS application (except it blocks the application that forces Safari to open the clips).
In practice, iOS does not give us the opportunity to choose what we want to open web links, send emails, watch any kind of video clips (whether they are available on the web or available in our iDevices) or let's listen to music. Apple has closed iOS so much that tweaks from Cydia are needed to have access to a set of options that other modern operating systems offer users without problems. I would like Apple to allow us to watch the content and the web with whatever we want, without being forced to use only certain applications, but maybe I'm asking too much...
Of course, jailbreak is always a solution, but not everyone prefers it.
