My guess is that they do it because they know most people in the higher-end hotels are on business. Therefore, they can tack on the $9.95/day + a bunch of taxes/fees and you'll still buy it and charge it back to your employer or client.

At the lower-end properties, they offer it for FREE as a competitive advantage. People at a $70/nt. motel aren't likely to buy 'Net access at the same level that a business traveler at Hilton would - but if they see a sign for it for free, they'll pull in to that Hampton Inn instead of one of the many other motel choices that are always nearby.

Probably the same logic that applies to hotel phone charges, although using a hotel phone to actually place calls is sort of a relic of a bygone era.