When you get out in the boonies, I think you might be surprised how many homes and businesses don't have FAST internet access. They may have semi-quick access, at least for their own use, but once you get beyond the reach of cable TV and get far enough away from the phone company boxes for high-speed DSL to be very iffy, it's far from a given. This tends to be the sorts of areas where many campgrounds are built, too.
Providing a robust, quick wireless internet setup that covers the several acres of a campground is not a trivial bit of infrastructure, certainly a lot harder than providing it for a dozen or two patrons in a cafe. It requires multiple access points, connected with some sort of medium area network (using wire or fiber runs, most likely). Getting a sufficiently large upstream connection to permit everyone to stream video simultaneously is likewise not a trivial or inexpensive job.
I do completely agree that, if WiFi is advertised as being available throughout the campground, it ought to be available and at least basically usable at all the sites. Where I disagree is that it's reasonable to expect it as a basic amenity on the same level as, say, having a picnic table and fire ring at each site.