As a Veteran and once again active duty, I don't believe there is any problem with getting a free meal. It's a company saying thank you is all. Now I don't seek out discounts and free stuff like a lot of people do (Applebee's is 5 mins from the house but I didn't go for a free meal just to get it) but if you're there why not?
I see no problem getting the free meal. It's their way of saying thank you for your service. I don't go out of my way to receive that free meal but I will go eat if in the vicinity or have time to. Semper Fi!!