I think you're overstating the case, things like: Large vs Small beer - http://www.youtube.com/watch?v=1xSDI9Gg63I - are flatly unethical. If someone asked for a 16oz Coffee and you gave them 12oz in a skinny cup so it looked taller that would be unethical as well.

But, if you have a series of smaller to larger cups lined up on a counter and you ask people which one they want and then give them what they asked for, I can't see that being unethical.

To me, this is just another form of advertising.