I guess they can... but the content of a directory can be quite broad in nature, and the process of listing a website in a directory so easily abused as to render it far less productive, IMO.
I just looked at your Legal Directory. That may be slightly different, since it is a directory of legal resources only broken down by location. If I was a lawyer and the price was right, I could see paying a fee to be listed...
That's correct about general directories. Its easier to have fresh relevant content in niche directories