Why should they, if they have to support H.264 encode/decode in hardware already? WebM hardware support would be an extra cost, and you'd better believe that they've got those parts spec'd down to the fraction of a cent. WebM offers nothing to Apple -- certainly not video quality, and they participate in the MPEGLA patent pools, so they're not paying for the licensing.
If WebM became a mandatory codec, they'd implement it, but they'd prefer not to.
