What if you drive a $30,000 car and I decide that I'd like to buy it from you. I appraise the car at $2500 but to show what a nice guy I am I'll pay you $8000 for it. If you aren't willing to take the $8000, should the government be able to force you to sell it to me for that price? After all, it's over three times the appraised value of the car.
Mind you -- I'm not on the government's side in this case . . . I simply wanted to point out that the property owners' legitimate complaint is one based on principle, not (necessarily) economics.
If I have a $250,000 home and I'm paid $500,000 by a government in a completely unjust eminent domain proceeding, I may be on solid grounds from a legal/moral standpoint -- but I'd have a hard time convincing too many people that my property was "confiscated" from me.