I've seen this practice the most from NY hotels. Nearly ever hotel has a bunch of towncars in front, and the bellman usually offers $50 or so to JFK. Of course, they fail to mention tolls, gratuities, etc.. so it works out to be nearly $70 when you get to the airport. I usually only use them when its rush hour and its hard to find a cab around..

I am sure the bellman gets at least 10-20% kickback, and won't be surprised if this practice is prevalant in other cities.