Great discussion here. As per my research so far, Myriad was initially designed for print media. Thus, the rendering is not that crisp when it comes to web. Apple being known for those little touches, hence opted for images with alt text (Accessibility).
I don't think the reason would be price/ anything related to buying it from Adobe as Apple being one of the richest companies, price would be the last thing to bother about.
But the next question was - WHY NO CSS SPRITES? After all, Apple being known for using multiple high resolution images per webpage, why would add unnecessary additional http request?
Answer is very simple -- Accessibility as there is no alternative text for background images.
Detailed Explanation -
CSS background images which are by definition only of aesthetic value – not visual content of the document itself.That is it – images in CSS are only visual extras, not page content, hence they never need alternative text. “Rounded corner” or “silver gradient” does not help anybody as alternative text – on the contrary, it annoys the end users.