Don't get me wrong, it's great that at least one more Android phone is getting pinch-to-zoom gestures, and it's never been clear why certain phones don't include the feature. Rumor has it Apple requested that Google avoid multi-touch back when the companies were cozy, but Google may no longer want to comply now that the relationship has soured.
We don't know the terms of the arrangement -- or whether it actually exists – so it's unclear whether Google could ever add multi-touch to older phones. For all we know, there may be issues with the Droid and G1 that preclude Google from releasing a simple update, regardless of whether Apple's involved.
Whatever the reason may be, the Droid and the G1 lack multi-touch for core Android apps, even though the hardware in both phones supports multi-touch gestures. This doesn't seem like an issue of older vs. newer versions of Android, because HTC's Droid Eris, which runs Android 1.5, supports multi-touch, as does Motorola's Milestone, the European version of the Droid running Android 2.0.
Nonetheless, the Droid's and G1's missing multi-touch is a clear example of Android's fragmentation. Android phones aren't all on the same page, with different hardware, user interfaces and versions of the operating system. A similar issue came up with Google Maps Navigation, which at first was supported on Android versions 2.0 and higher. Droid Eris and G1 owners eventually got the update that makes navigation possible.
It's a shame that you can't purchase an Android phone knowing for sure that you'll get multi-touch, or when a new feature such as Maps Navigation will be brought to your older phone. The flexibility of Android lets users decide on the hardware and user interface that's right for them, but that choice comes at a cost.