GSM have no problem with non-GSM sandwiches on the EDI/AGP & GLA/AGP flights.
I accept however your post was probably not about these routes.
At the end of the day you are sitting in a seat on a no-frills airline. Presumably you would not be there if you thought the ticket price was too high. They've got to make some return on the passenger other than the flight ticket price surely. I don't see anything immoral or unusual in this especially when, you clearly state, they are telling you the score Re food upfront.
You pays your money..........