Declaring your firearms makes perfect sense to me, in one respect: Making sure the firearms are unloaded. How many dimwits have you heard of/seen taking loaded guns to sell/have repaired at a gunshop?
As far as the guns being confiscated, though, I'm not a fan. I believe a fine should be imposed, fitting the crime. They found a gun you didn't declare. On average, this costs the TSA $XXXX.XX to carefully open your baggage, ensure the gun is not loaded, do the proper paperwork, etc. That's what you're paying. Simple confiscation... don't like it.