When you buy a ticket, there's a "view fare rules and restriction" link. When you click on it (I'm sure you didn't), you're prompted to promise that you're not a robot. Then the fare rules and restrictions (all of them, including change fees under "voluntary changes") come up. You can certainly be excused from not looking at them (few people do), but the terms are there and you do indeed agree to them.

Also, just after select payments, there's a "Terms and Conditions" box with another link to the "fare rules", where once again you can look at all the rules and change fees. Again, few people actually look at the fare rules and conditions they're agreeing to, but they're there.

It would be nice if UA would just put the change fee on the receipt.