Because that's how hotels make their money. I'm afraid you'll have to get over it. Or use an internet cafe.
I don't think there are many hotels that supply free internet access. I've stayed in hotels from five-star palaces to basic backpacker places and I have never had free internet access in any of them. I'd be interested to hear of any that do offer free access.
And why shouldn't they charge you for a service they provide? It costs a hotel a lot of money to run a network and provide internet access to its guests. You wouldn't expect a hotel to feed you for free or to put you up for free - why should anything else be free at a hotel?