Er, they are, but they're opt out rather than opt in. For anyone committed/proficient enough to use the SDK, opting out should hardly be a big deal - four mouse clicks.
I am going to shoot the next person who says that because it is DEAD WRONG as I have written time and time again on these forums. There is no way to turn off Steam's automatic update checks and downloads that occur each time a game is launched. You can disable the background update checks and downloads which occur when Steam is idle (which is what that setting does), but not the updates which occur at each game launch if Steam is not in offline mode.
Though what you'd use the SDK for without Half Life 2 I don't know.
First, you can edit any source engine game with it. Second, many modders including myself don't always have games installed on a PC with the editing tools because the games take up so much room. I like to use a separate PC for editing especially since it usually doesn't even have to be able to run the game to use it for modding purposes.
It's like saying you'd like to be able to use a mouse without having a computer as far as I can see.
That ridiculous analogy explains a lot about the rest of your posts.