miko,
You are right: the first amendment doesn't make a distinction. The distinction has been made or defined in many court cases since then. I'm not a legal scholar, so I can't cite anything off the top of my head, and don't have the time to do a search at the moment, but this is something I've read about many times.
I can give you an example:
Those politicians' signs in the along roads around election time? They are legal.
Try putting up a similar sign in the same place advertising your product or services and it will be taken down. Illegal.
[I talked to a police officer about this once.]
However, many communities allow real estate signs to be put up since people need to be able to find homes for sale. It ain't fair, but that's how the law has been interpreted for some time.
Gotta run.
MRPLUTO