Blizzard didn't even package the proper tools with the game. They used a 3DS Max plugin that they still don't have for the rest of us to use, and while they could have used the Galaxy Editor to make their campaign, they didn't. This was also true for WarCraft III, except they let us use the damn plugins required to model.
They didn't ship it for us because it's not their program to sell. If you want to, you can buy it off of their website, though.
..No, you don't understand. You need the plugin in order to make models compatible with SCII (at least, you do if you don't want to do any roundabout way that I don't recall).