Correct me if I’m wrong, but won’t all of the apps you mentioned run on a Mac? You have to load Windows for Paint.net, but the rest will run in OS X.
And a good portion of iPad apps are probably free. There are a ton of free iPhone apps. I don’t know how the ratio would breakdown and how it would compare to the ratio of paid vs free for desktop OSes, but there are a good number of free apps out there.
You are correct. The Mac is the last true general purpose computer that Apple produced. All the latest i{Phone,Pod,Pad} gadgets do not permit you to load and use a program that Apple hasn't approved. Current rules also demand that such an application must be handwritten for the gadget, and not compiled or translated from another language. This puts a major crimp on any porting desire that developers may have.
And a good portion of iPad apps are probably free. There are a ton of free iPhone apps.
Some would be free, but they still have to come from a developer that is authorized by Apple, and he will need a Mac to write the software. The costs aren't that high - a $99/yr for the iPhone developer program and whatever you can spare for a Mac. There is also a free developer program, I don't know how it compares to the $99/yr plan and what are the differences - Apple doesn't say and does its best to hide it. To compare, I downloaded the Android SDK without signing anything or paying anything, directly from Google, and there are tons of resources to help to get you started (they are pretty good.) Some free software gets started as a weekend project, and then they get life on their own. Having fewer obstacles translates to more software.