I tend to side with the OP. If your website says you have a shuttle, how much more trouble is it to post the hours it runs or any other restrictions.

I quote from the ATL Airport Hilton website:

Our courtesy hotel shuttle service is available in the Ground Transportation area west curb, just immediately outside the Airport between the North and South terminals. Hotel shuttles are located in the Blue Bus aisle. The shuttle departs every 15-20 minutes from the hours of 4:00 am to midnight and is on call after midnight. Courtesy phones are available at the airport for after hours.