The [url="https://us.battle.net/support/en/article/starcraft-ii-system-requirements"]system requirements[/url] when HotS first was available for pre-purchase included 10.7+ are the only supported OS.
The [url="https://us.battle.net/support/en/article/mac-os-x-support"]Mac OS X Support Policy[/url] pretty much outlines reasons why an OS may not be supported in the future.
On the day that the pre-orders were made available, the system requirements pages were directed at the Starcraft 2: Wings of Liberty system requirements (at least at the time of my viewing of them and my pre-order). At that time, 10.6.8 was listed as the lowest supported OS. It may very well have been the case of someone not updating the linkage at the time. My opinion on this is that it should have been made more clear. At the time of pre-order and prior to this post it seems pretty clear that the beta did indeed work on 10.6.8. The fact that you are dropping support for this OS in your NEW title isn't as much of a concern as the fact you are dropping it from your OLD title on the whim of a new engine or graphical options. While it might take extra time to add in toggles to disable these new features to WoL, there shouldn't be a reason why you drop your shipping platforms while it can still run on them just fine.
I would like to explain a few things and I understand what you are saying.
Okay heres a few details which the blizzard CMs haven't gone into in this thread.
1. WoL and HoTS, will be 1 Client, With an option to switch between expansions so both will be on a single client, so therefore both will HAVE to have the same framwork.
I. This helps make it so you dont have a 32 Gb folder of just starcraft with duplicated models, since HoTS will be modifying MANY pre made models, all the way down to the marine adding rag-doll. So this will keep the files a bit smaller. Which in turn will help all mappers so we can put our maps with varying mods on Both WoL and HoTS, this also allows future updates to the engine. But unfortunatly things will have to be dropped, and since 10.6 runs on old openGL, thats one of them.
Luckily its cheap to upgrade from what I hear, like 20-30$ is what people are saying.