If you make an invalid fair catch signal, you are fair game to be tackled. However, players are still coached not to hit the opponent in that situation, because with the opponent likely standing still, teeing off on him runs the risk of drawing a flag for unnecessary roughness even if tackling him in general is legal. Also, the officials are supposed to blow the play dead as soon as the ball is caught, and if you tackle someone well after the whistle, you can also get flagged. So while NU's players could have tackled Erickson, they are taught not to, to minimize the risk of needless penalties.
I see the point of avoiding a possible penalty. But it is still unclear to me what are players taught to do for a situation in which there is no whistle and the return man -- after making some type of waving -- stands with the FB in his hands, by all indications getting ready to take off. Are they taught to wait until he takes the very first step? (that wouldn't be a long wait anyway, unless the returner for some strange reason freezes and stand still).
As I have indicated before, in the case in question 4 unblocked NU players were 5-10 yards away when the return man was in position to take off. So, they had a bit of room to maneuver.