Gnorky, when Hellfire was released, that was the REASON they didn't support it.

I am not just guessing here, that's what was said.

Sierra and Davidson and Associates were together, Sierra wanted to do an expansion pack for Diablo. Blizzard didn't want to. But they were forced to let Sierra make it anyway, and publish it.

So it comes out as officially supported but non-official. None of the story has ever made it's way into any other Diablo product. And because they did not develop it, Blizzard would not let it work on Battle.net because they couldn't guarantee it's quality. These are all reasons explicitly put forth when it came out. People wanted it on Battle.net, but Blizzard said sorry, no.

Find a single mention of Hellfire anywhere on Blizzard's site. The only thing you are going to find is that it's not a Blizzard product, and is not supported by Blizzard. They distanced themselves from the game, because it was not their game.

A company comes along and says "Hey, we want to do an expansion for your game. You can't deny us because your parent company owns us. And we want it to be compatible with your online network." see how well that goes.