It's not a surprise because Apple has discontinued every previous version of OS X once a new one has been released. The only exception that I'm aware of was 10.6, as it was a prerequisite for 10.7.
Before I left for work Lion was still in my purchased list. Didn't try to download it but if it was pulled how come it is still in my purchase list? (Note: I have not bought ML yet. And yes a real question not a sarcasm statement.)