It depends on the size of the plane. Ezy's 319/20s don't have a problem with it, but 321s do. We have to security check each and every seat before the first flight of the day and during every turnaround, even if it has not been occupied. After take off the restriction is lifted, and you could move to these seats if you wish. I believe that airlines with allocated seating deal with this issue at check in. I think Ryanair do it because they have 737-800s which are quite long too.