Seems everyone is chiming in with their own opinion so here is my thoughts.

I believe the real culprit in this case is the definition of a term not keeping up with common every day usage. While I personally wouldn't care or expect an actual piece of steak of a ~$3 sandwich, I know some people might. I think the word steak deservers a more cleanly defined meaning. Something akin to "A single slice of meat from such and such part of a cow." I'm not an expert on the legalese for it, but you get my point.

I will say in closure, that I'm not sure this is something that needed to go to the courts over. If steak and hamburger where different species of animal I think it would matter more, but the better thing to do in this situation I think is to either:
A) Just not ever buy one again. (The answer I normally go with.)
or
B) Ask Dunkin Donuts for a refund and then never buy it again.