As an airsofter, I wouldn't say it's done out of malice. They probably want to make sure that someone cannot potentially modify their velocity by adjusting their hopup - you'll find that chrono limits are often specified with "Hop Up" off as the hopup can affect the velocity and thus energy output of a gun.
This is in part due to several things - their insurance coverage, which specifies their allowable energy limits, the desire to treat everybody with fairness (After all, it would suck following the rules, only to be shot by a hot gun.) and lastly, the enjoyment of airsoft that would encourage someone to come back.
So, the best thing I can say is, mark where you left your hopup (Silver sharpie is amazing for this) and adjust it back when they're done with it. I'd rather adjust my hopup again than potentially hurt someone who's just looking to have a good time airsofting.