Since Apple has the only mobile platform where you can only get applications from one source (them), this is really simple.

For instance on my Windows mobile phone, using MSIE mobile (which sucks) I can go to opera.com, download a cab-file, install it and I'm good to go. Google maps? Just go to maps.google.com and it will offer you to download and install a native Google maps client.

If I want to, I can develop stuff myself, no signing or service-provider or Microsoft approval needed. What exactly are you implying here?