Very good question. USAir customers wondered the same when they started hiding a $5 fee a few months ago. I don't think they ever announced/acknowledged it, and I think it's now gone.

I suppose from a revenue maximization standpoint, there's an argument to be made for quote the "ticket price" very low to drive customers to your airline, then tacking on "fees" to make the net cost the same as others. Pretty unethical though.