It's immoral because you don't have the right to deceive and make decisions for other people.
Let's take the example further and see if you can find a point where it starts to be obviously unethical, then work your way backwards on the principle discovered.
How about we lie to you about the contents of your food, in some way that manipulates you to be healthier?
Still OK with that, why don't we tell you that something is vegetarian if you can never tell the difference, by your utilitarian metric you've increased my happiness, I got to enjoy some delicious meal.
If I find out you have one day left to live and that day will be happier not knowing that fact, do I have the right to lie to you about it? The utilitarian benefit calculation goes up so case closed right?
If someone if a devout Jehovah's witness, give them a blood transplant and deny that it ever happened, do you have the right to do that?
Not everyone is on the same utilitarian value curve as you and you don't have the right to mess with and deceive them because you think you're smarter than they are. Some (most) people value truth, dignity or reality in their utility function, and you might not even be smarter than all of them.
I'm sorry but I don't buy the slippery slope argument here. No one is saying that the ends justify the means. Is adding a stop sign to a random intersection any different? Who decided that people should stop there? In small towns, a lot of times it is because an elderly person went to the township meeting every week for years complaining so they gave up and added one, not because there is any inherent safety issue.
Governments and corporations literally lie to us every day. Not causing a light to change three seconds earlier (because it will still change) doesn't violate anyone's civil liberties or religion or cause their death. Governments do manipulate things already to make people healthier and safer. Soda taxes, seat belts, food pyramids (bought by the FDA) are all examples.
In this case, most municipalities have laws against jay walking. One could argue this is just an enforcement of that law.
If action X is unethical/immoral in some situation S, this in no way implies that for all situations, X is unethical/immoral. See killing people for an example, perhaps, or maybe telling people they are (not) ugly?
Let's take the example further and see if you can find a point where it starts to be obviously unethical, then work your way backwards on the principle discovered.
How about we lie to you about the contents of your food, in some way that manipulates you to be healthier?
Still OK with that, why don't we tell you that something is vegetarian if you can never tell the difference, by your utilitarian metric you've increased my happiness, I got to enjoy some delicious meal.
If I find out you have one day left to live and that day will be happier not knowing that fact, do I have the right to lie to you about it? The utilitarian benefit calculation goes up so case closed right?
If someone if a devout Jehovah's witness, give them a blood transplant and deny that it ever happened, do you have the right to do that?
Not everyone is on the same utilitarian value curve as you and you don't have the right to mess with and deceive them because you think you're smarter than they are. Some (most) people value truth, dignity or reality in their utility function, and you might not even be smarter than all of them.