Wow.. That's something you'd hardly believe. I bet that lady got a major scare!
The reason for that is to prevent explosions. If you're sitting on a seat it's possible to create friction by swaying back and forth on your seat. This would trigger a spark and an in turn an explosion.
http://www.youtube.com/watch?v=tuZxFL9cGkI
<-- Link to a video where it shows how static electricity can cause fires from sitting back on your seat.