Why can't I install TF2 in a non-default game library location?

I've set up 2 non-default game library locations. One in C:\Games and the other D:\Games.

Steam is installed on C: so I have 2 libraries on C and one on D.

When I go to install most games it lets me choose, but not Team Fortress 2. The dropdown to choose library isn't there.
I'd understand if I didn't have space, but I have easily enough space on C and D. Besides, it should at least offer the C:\Games location as that's the same hard disk as the steam install library, which it uses if I continue without being able to select any.

There are a few other games I can't install to a non-default location such as Portal (orignal) all the HL2 games, L4D2. It looks like it's old source games that won't let me choose.

TF2 uses the Source engine, as do several other games by Valve. To conserve disk space by sharing files, Steam automatically installs all of them in the default location. There is no way to change the installation library folder for these games.

If you want, you can move the files manually and create a link (in cmd.exe).

For example:

mklink /J "C:\Program Files (x86)\Steam\SteamApps" "G:\Steam\SteamApps"

That will do the whole steamapps directory. If you want to do pergame, then you will have to move directories around manually which would be a major pain.