The iOS developer fee exists for exactly the reason 
newt specified. $100 a year really isn't a huge amount of money, but it acts as a deterrent (mostly) for people who would seek to abuse to the app store, and for those who aren't serious about making apps. Contrast that with the Google Play store, where anyone can release an app. That place is a quagmire at times, full of misleadingly named apps that mimic genuine apps but for nefarious purposes such as password phishing.