You don't, but enrolling in any kind of developer program, or buying any kind of developer software, is going to cost north of $99 a year, which any developer hoping to make even that amount of money will do.
The situation for iPhone developers have a lot of well publicized flaws, but the cost of participating in the developer program (which includes development software and publishing in the App Store) is not one of them.
I develop .NET I can use notepad and csc for free or I can buy tools from MS that save me time. Halfway house is SharpDevelop which is free but not as good as Visual Studio. I don't mind paying for cool tools but it sucks to have to pay to release software.
The situation for iPhone developers have a lot of well publicized flaws, but the cost of participating in the developer program (which includes development software and publishing in the App Store) is not one of them.