Is it necessarily the spirit of bribery that the meals are offered in? "I'll scratch your back" does point in that general direction but the intention of the owner could be more of a "do something nice for those guys who risk their lives for us for too little pay" like places that offer a military discount.
Every business I have ever had that offered me a discount I have always had the impression it was a gesture to show that they apperciated us and wanted to do something to thank us. It almost feels rude on my part to refuse it because of this, but I like my job very much and would not like to loose it over the matter of a free meal so I always try to politely tell them that I appeciate the offer, and thank them for the gesture, but my department policy does not allow me to accept such gestures and that I could get in trouble for accepting it.
There have been times that business will refuse to allow me to pay even after explaining this. In these circumstances I leave a tip large enough to cover the expense of the meal, or as many businesses have a charity donation recepticle place the money for the meal in that and let my sergeant know. Unfortunately because if the uncomfortable position these business put me in, I usually do not return, so they end up hurting themselves by taking this approach.