potentially costing the airline money if the flight is busy
It costs the airline nothing if you don't show up to occupy your seat, in fact there would be a marginal fuel saving resulting from not having to carry you and your bags.
Yes, they are arguably foregoing potential additional revenue, but that's something completely different.
I had a similar experience coming back from Cologne on Germanwings a couple of years ago - my business finished early, so I asked to fly on an earlier flight (on which there was loads of space), which would have enabled them to re-sell my seat on the (full) later flight at a much higher cost than I had paid. The ticket agent agreed that it made sense, but said his hands were tied.