Should the back of the goal nets be pegged down?

I like sitting in my seat before games and watching the warm-up and kick-in. It would appear that when balls are hit into the back of the match nets, the 'frame' round the base of the net lifts a little? It seems to be happening in both goals, and makes me wonder if this is not in compliance with some rule? There is the possibility that in a goal-mouth melee, a player might end up in the goal, accidently lift the 'frame', and a strike into the goal may not register, as the ball could just go under the raised, unpegged, frame? Just a thought like...

Believe in a palace game number of years back (v Bristol city if I remember correctly) the ball hit frame at back of goal, bounced back out and no goal was given.