We all surely agree that lying is immoral and, most of the time, a terrible thing to do. We all surely agree that lying by omission is equally evil, most of the time. As we say here, a lie is the theft of somebody else's reality.
In life, we tend to identify liars and to distrust them, figuring reasonably that if they lie about one thing, they just tend to be liars. It's not always true, of course, but it's a safe rule of thumb.
Robin Hanson asks Why Allow Lies? He says:
Claiming military honors offends a lot of people in a way that claiming honors for other subjects doesn’t. Personally, I think that outlawing things merely because they offend people is a bad idea (and unnecessary–you’ll note that the offender in this case got his comeuppance even before the law was applied), but that’s another argument.
Making lying illegal seems crazy to me. For starters, every politician would be convicted.
Here's Lex's take on Stolen Valor.