$100 can be a lot but it's not $100/app/year, right? For $100/year you could have several applications?
It's not a lot given what you get for the money (easy app publishing on the App Store, call support with Apple engineers etc.).
But SwiftQuit's dev doesn't make money with the app so he didn't see the need to pay that money just to sign the app. An unsigned app downloaded from the dev's website is as good as a signed one.