This is considerably different, being opaque. Once this gets out, everybody taking an Uber will have that nagging feeling in the back of their head, "was I ripped off this time?" And it'll make them think twice the next time they go to order one.
People act very emotionally to the idea that someone else got a better deal than they did.
It could just as well be that the "higher, weekend" price is the "normal price" and the weekday price is the "deal, sale" price.
Think about it this way: You would probably pay a hospital your net worth if you had a child and he/she was seriously hurt and taken to a hospital. B would you think leveraging this information to take more money from you be "fair?"
If you don't like what Uber is charging you, then don't take Uber. You can consider whatever constraints you like when determining the price you're willing to pay: convenience, time, cost, cleanliness, location, whathaveyou. Maybe you value your time less than the cost of an Uber, so you can then walk to your destination. You ripped off Uber then by using your two perfectly good feet, and showed those capitalist fatcats by not even using their service!
The comparison to a serious hurt child is not an appropriate comparison (also, "think of the children!"), because the service is not fungible in that case.
Is someone going to show up and make the same arguments for why people with higher incomes shouldn't pay higher taxes? If there's nothing wrong with progressive taxation, then there's nothing wrong with progressive pricing, and the only difference is that you're forced the pay taxes while you're not forced to purchase services from any given vendor.
And I'm not arguing this from a purely economics perspective. Yes, I know discriminatory pricing has its place in our society today and yes, I know it's profit maximizing, "fair," etc. But what's different here is that unlike airlines that have an algorithm deciding the same price for anyone buying that seat at that particular time, Uber can discriminate against you for very personal reasons, or general reasons that can discriminate against the racial makeup of a neighborhood, the disabled, etc.
Hats off to your sense of duty to capitalism that'd make Milton Friedman proud, but the world doesn't work like an Econ 101 textbook. If Uber's profit maximizing algorithm is discriminatory against protected classes, it's illegal. If it's profit maximizing by overcharging people lulled into usage due to habit, dying phones, etc. it's still predatory.
P.S. regarding your "think of the children" critique, I'm simply offering a direct counterexample to your "whatever price you pay and agree to == not getting ripped off." According to you, it does seem like you can be ripped off after all, even at a price you were willing to pay.
In the hospital example, if you had a choice of several hospitals at different price points and chose the more expensive one, then it is fair of them to demand the price they quoted upfront.
Likewise, if the hospital needs the money just to cover the expenses incurred in trying to save the child, the price is fair as well.
But if they have a choice and you don't, they are unfairly abusing the situation. (See also: Martin Shkreli jacking up the price on a drug exclusively manufactured by his company.)