I don't see any reason in why Mojang didn't add the new slabs as 43/44:7 instead.
Mojang has never done this before (replace block IDs and break existing builds) - I don't understand why they did this time.
When flammable wooden slabs were added, the pre-12w17a slabs were still kept as they were and the new slabs were instead given a different ID.
Why not the same with Netherbrick Half Slabs?
Never mind - I just reset my .jar and this has cleared up.
How can I delete the post?
They reset along with the minecraft.jar, it's not really a bug.
But I do believe that this should be fixed, though.
Then how come clay and snow blocks can be transformed back into their base materials?
Also, Quartz Blocks are crafted from Nether Quartz, which is an ore like from any other mineral storage block; they should be able to be crafted down as well.