I actually agree with you F3G. I would have been highly peed off if it had been me - probably more so as I pay for C class tickets out of my own pocket. From your (and my) point of view they got it seriously wrong. Unfortunately, money is so tight these days that from their point of view they saved themselves a load of dosh, albeit at the expense of one or two unhappy pax (too many do-gooders and sheep out there who wouldn't bother to complain). You may well find that they will compensate you because you went to the trouble to complain.
I had a slightly similar type of thing happen to me some years ago with Cyprus Airways when they substituted a Eurocypria all Y configurated A320 instead of their own CY configured A320 on a BRU-LCA scheduled flight. We still got the C service but not the (dedicated seats) C comfort as advertised on their web. It took several patient (on my part) exchanges before they finally conceeded that they had not provided the promised ADVERTISED product and I was compensated. I bet I was the only one among the 10 or so C pax who took the trouble to complain. Most businesses won't offer compensation up front if they can avoid it. They'll just wait and see.