I would think "It's a trap!" and I wouldn't go for it. There's just something about eating food that was on a stranger's table that doesn't appeal to me. You don't know what's been done to it.
But morally speaking, I see nothing wrong with eating it if it was left there. It's already been paid for and the one who ordered it obviously didn't want it. So as far as I'm concerned it's up for grabs.
The only problem I foresee the restaurant having is that they might not let you eat it for legal reasons. If in fact it was tampered with and you ate it and somehow got sick, they'd have a potential lawsuit on their hands. If they don't let you eat it, it's because they have to cover their asses as a business.