That, combined with the fact that, yes, businessfolk will just expense it, leads to the current situation.
The answer is really, "because they are rapacious".
I used to work for a fairly large hotel chain that would charge ridiculous amounts for internet. Thinking back it wasn't too ridiculous (certainly not 10k for 3 days as someone said above) but it was still a high price.
The only people to really use the service were business people, and they'd always charge it back to their companies.
The WiFi itself was merely a luxury we offered to encourage business people to stay in the properties because "WiFi available" made for better advertisements.
Heck, I know one convention center that charges $1500 just to hang a small banner over the door of the ballroom with your event's logo on it.
http://www.reddit.com/r/AskReddit/comments/e2uui/why_is_wifi...
As long as they clearly label them as a cost+ and provide you those dinky plastic cups you can drink tap water with, I am happy.
3 euros, 3 pounds sterling, or 3 dollars is common. I guess the US wins in the "over-priced-water-in-your-room" war. There must be something magical about the $3 price tag.
Much more exciting have been the freebies (zero status/independent hotels): chocolates, biscuits, wine, platters of local delicacies, fruit baskets, free non-alcoholic mini-bar replenished daily... :-)
Ironically, they gave me at least $5 worth of chocolate chip cookies when I checked in.