Can it be that, at that moment when you made the decision, web sites 'made for internet explorer', were too many. And you wanted the rendering engine on Mac to be widespread by getting from and giving to an open source project.
Remember, Safari is not open source. Only WebKit is.
2. iTunes, at least in the beginning, was just a wrapper around the QuickTime framework.
There's more to it than that, but this touches on something; I really wish that something akin to QuickTime was available cross-platform. Yeah, it's crufty and strange warts that reveal Ugly Things are all over the place, but man, having a good AV API would make a lot of things better.
ffmpeg is a very impressive achievement, but it's not the same thing.
Had KHTML been licensed under a BSD or MIT license, the browser world could potentially be very different from what it currently is.
Next, Apple was only able to negotiate licensing deals for music (and later video) content from the various rights holders after agreeing to various strictures including sufficently restrictive DRM and functional restrictions such as allowing music to be copied only from computer to iPod and not from multiple computers to a single iPod.
Apple didn't try to stop third parties from working around some of these restrictions but it couldn't be seen to be allowing them to be circumvented willy-nilly and open-sourcing iTunes would immediately lead to a "rip everything to MP3" and "grab everything from all attached devices" features.
Regarding iTunes, Apple wouldn't want anyone to release a competing program. You could probably outsource some common libraries/components, not sure if this is done. But anything specific will not be used or improved upon by others anyway, so there would be no benefit in opensourcing it.