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.
Being in Patents Pools doesn't mean they dont need to pay licensing. It just means they are on both paying and receiving ends. Since there are thousands of patents on H.264, Apple are only recieiving a tiny amount compared to what they paid out.
If WebM became a mandatory codec, they'd implement it, but they'd prefer not to.