I don't believe they ever said it was native - it's simply a Lens app, which means you can launch it straight from the camera.
For some reason it's not available for WP8 yet, anyway - I'm guessing the WP8 version is still being finalised.
Imagine if everybody new to WP who just picked up a Lumia or 8X installed the Photosynth app and was presented with the WP7 version and no Lens capability - they'd probably wonder why it doesn't do what was advertised. Better to just say "it's not ready yet" I think.