The argument is we wouldn't need their marketplace and services if they allowed users to easily download and install software downloaded from regular websites.

you know... the way it always worked and still works in Linux/BSD.

There are obviously security implications to this, but I believe if I own the device I should be able to take the risk.

As far as their SDKs they actually do need to provide those for free, otherwise nobody will develop software for iOS and they won't be able to sell devices.