I can because Apple allows them to exist in its ecosystem. It can just as well choose to disallow them. Here is an example from a couple of years back: http://www.apple.com/hotnews/apple-answers-fcc-questions/you can use Chrome with the iPhone and iPad as well as Safari which is the default browser.
"Question 1. Why did Apple reject the Google Voice application for iPhone and remove related third-party applications from its App Store? In addition to Google Voice, which related third-party applications were removed or have been rejected? Please provide the specific name of each application and the contact information for the developer.
Contrary to published reports, Apple has not rejected the Google Voice application, and continues to study it. The application has not been approved because, as submitted for review, it appears to alter the iPhone’s distinctive user experience by replacing the iPhone’s core mobile telephone functionality and Apple user interface with its own user interface for telephone calls, text messaging and voicemail. Apple spent a lot of time and effort developing this distinct and innovative way to seamlessly deliver core functionality of the iPhone. For example, on an iPhone, the “Phone” icon that is always shown at the bottom of the Home Screen launches Apple’s mobile telephone application, providing access to Favorites, Recents, Contacts, a Keypad, and Visual Voicemail. The Google Voice application replaces Apple’s Visual Voicemail by routing calls through a separate Google Voice telephone number that stores any voicemail, preventing voicemail from being stored on the iPhone, i.e., disabling Apple’s Visual Voicemail. Similarly, SMS text messages are managed through the Google hub—replacing the iPhone’s text messaging feature. In addition, the iPhone user’s entire Contacts database is transferred to Google’s servers, and we have yet to obtain any assurances from Google that this data will only be used in appropriate ways. These factors present several new issues and questions to us that we are still pondering at this time."
You can download apps that modify every behavior of Windows. There is no policeman like above stopping you. To wit, there is an app that gives you the Start button back in Windows 8.
More examples: http://en.wikipedia.org/wiki/Approval_of_iOS_apps
"Applicants have been denied apps for fundamentally duplicating Apple apps, for example by providing an email interface with no added functionality or use,[8] or having the same elements as the built in SMS app.[9] Applications have also been rejected for duplicating the functionality of iTunes.[10] Applications may be rejected if they are of only "limited utility".[11] Some reports indicate that toolbars must be placed at the bottom of the screen, and the vibration function should only be used for alerts.[12]"
During the last few years I have had this professionally impact me in dealings with ideas of new start ups that we could not create because we knew it would require Apple's approval and they would not give it.
Apple has become more flexible over the years but still has a stronghold on its users. One example is their requirement for 30% cut to go to them for any in-app purchases. Such a measure completely disables the ability of anyone for example to sell you movies and music as neither has that kind of margin in digital world. Yet apple can do it since it doesn't have to pay such high mark-up.
Sadly others then copy them so the impact is much larger on us as we move to mobile world. We are losing freedom we take for granted in open PC/Internet world.