I guess it's like an entrance fee to stop the club getting overcrowded. Apple Developer support forums, apple developer support, the App Review process...all these things take resources and are probably already overloaded (or pushed to limit). The fee helps keep check of that, like charging people for your side-project development work. Sure, you could do it for free (as you have a main income) but charging clients helps you manage and prioritize your workload.

Incidentally I've made back my Apple Developer fee in app revenue, so that's cool. Ha ha ha! :)