Presumably this would result in a flood of "free" apps that are nothing more than facades for scams (e.g. a fun personality/quiz app -- look ma, no ads! -- that makes you reveal your secret questions and answers).
Maintaining the integrity of an app store is a very non-trivial task, and building an entry barrier in the form of developer fees is one way of doing it. Not saying this is the only solution or even a particularly good one, but every solution will have its share of unintended consequences and exploited armor chinks. Making dev accounts free for no-ads would be an interesting experiment, though. I'd support it for science!
Ha, that's a good point, though I think if Apple were to implement something like it, there would be a ton of rules. Perhaps banning network requests from the free app (no saas, no tracking) or limiting it in some way from being abused.