I don't think it's that bad a move, but will likely cause them to lose some business. I've bought a couple tickets before with the hope of catching earlier, more expensive return flights, but those turned out to be full. So I happily flew on my original one (I wouldn't have bought it if I wasn't willing to). When flying leisure, I wouldn't pay $100 for an earlier flight.

I wonder if a flight is oversold and they want to stick me on an earlier one, if they'll pay me $100 more now?