AFAIK axle straps were to limit axle travel and stop early dampers that had no internal limit buffers fitted bottoming out,
AND to prevent military vehicles driven by private Gunga Din getting too deeply stuck and requiring the whole REME regiment to dig them out.
Agreed, they are there for a purpose. When fitted they should obviously have some slack to go round the axle but may have to stretch a little at first, but there should be enough slack to fit them. To get some stretch introduced it depends on where you are driving. Off road will soon stretch a little if going over rough ground, or on road if you hit the odd pot hole!
They're meant to only touch when your shock absorbers are running out of travel. I got a Rocky Mountain springs and shocks kits and they seem pretty adamant that they should be fitted!