Restaurant rip off - what do you think?

I received an email solicitation from a local restaurant indicating "$5.00 off every adult brunch" which sounded like a good deal. Since we were late getting started we selected to order off of the "Brunch Menu" vs. enjoying the buffet as it was close to 2:00 p.m. and it appeared they were starting to break down the buffet. Our meal was excellent. We presented our server with the coupon and he indicated that it was only good for the buffet. After pointing out that it indicated "brunch" not "exclusively the buffet" he said he would get the manager.

The manager arrived table side with our bill and in a very condescending manner indicated that he had taken the coupon off our bill, however, it was meant for the buffet. We attempted to point out that no where in the email did it indicate "buffet only" and since they have an exclusive "brunch menu" it would appear that the coupon was valid for anything on that menu. His behavior was that of annoyance and we should be grateful for the favor. We conceded at that point and felt it was a mute point to even make him aware that the coupon was for "$5.00 off every adult brunch" and he had only taken $5.00 off of the one meal.

Are we being unreasonable?