I understand the math perfectly but 100 bucks for a year isn't a large amount of money and it is required to pay it only when one wants to deploy a project to the app store, ad-hoc or enterprise (199$ here) release. For anything else (as it seems you already know) like accessing the documentation and learning or building and testing an app in the simulator is free.
So, I strongly believe that those 100$ are not making any meaningful difference.
Here is math versus human psychology. Why? Because a Joe that thinks it has an app that could make him rich, will pay those 100$ whatsoever and regardless of what others think about his product (being a crap actually). Not to mention that some of them, would pay it only for the thrill of submitting an "app" for review and / or the "honour" of being an "iOS Developer".
Sure, if those 100$ would transform into 500$ or 1000$ (hopefully not giving any bad ideas here

) it would become a decisive factor and a great barrier for crap apps and of course for the volume of the apps submitted for review.