Fair Catch Signal

[jmctigerpaw]

In the NFL rules:

If a player signals a fair catch, is any member of the receiving team allowed to advance the ball. In the St. Louis Rams game Sunday:

Rams punted
Cinci signals fair catch
Ball bounces over his head and into the end zone
Rams touch the ball, but it isn't blown dead
Cinci picks it up and runs it out of the end zone for about 60 yards
(play was called back because players from both sides entered the field before the play was over)

However, wouldn't this have been an illeagal fair catch signal or in the NFL are you allowed to advance the ball after a fair catch signal has been made? Just wondering what the NFL rule was on fair catch signals.

Thanks for any clarification.