If you dig déep enough you will find that your seat is simply a request like a special meal. Nowhere in the travel industry are you ever guaranteed the hotel room,seat,flight or rental car. Essentially you are renting a space on the flight which is subject to change due to mechanical issues.

For me I know that when I book my seat it is subject to change up until the moment the plane takes off. As long as I get a comperable seat or better then I am fine.