the PRE OP refraction they refer to is only the SPHERE AND CYLINDER they dont mention that you can't have lasik for ASTIGMATISM
I Only just realised the above NOW...
The reason for setting a limit for refraction for SPHERE is due to MYOPIA (short sightedness) meaning that lasik doesnt take the risk of RETNAL DETACHMENT away!
Retinal detachment risk occurs in everybody with myopia as they have a longer shape eye which increases risk of the retina peeling away - this is a major ophthalmic emergency.
Obviously you can go blind in minutes not hours if this happens and what if you were on a transatlantic flight ?? i think this is why the limits are imposed!
JONATHAN