Why does Apple's website not make extensive use of CSS3 fonts?
Apple has been notably touchy about 3rd-Party use of their fonts in the past and I'm inclined to say they still are.  Apple use to include a few fonts in OS 8, for example, which had Apple-copyrighted design.  You had access to them in that anything that specifically used that font rendered correctly, but if you wanted to be able to select the font in a text document, you had to jump through some hoops and enable it manually.

The font in question is almost definitely Myriad Pro or an Apple derivative.  You "must" purchase a license to use it commercially.

While I generally agree with your assessment of using CSS3 fonts, it seems that Apple may be more concerned with copyright.  Load time differences are probably negligible for most people accessing the Apple site.  I'm sure they have people in Marketing looking at that data.