How was it forced? Steve Jobs: Let me just say it: We want native third party applications on the iPhone, and we plan to have an SDK in developers’ hands in February. We are excited about creating a vibrant third party developer community around the iPhone and enabling hundreds of new applications for our users. [1] Oct 18, 2007

[1] https://discussions.apple.com/thread/1183763?start=0&tstart=...

The iOS SDK was always going to be released, but only for the (then-unannounced) iPad. That's how they were able to change their tune so quickly when users demanded more than web apps.