Which is likely to be in response to RIFs being used in the commission of crimes that attract press attention, not being sold quietly to airsofters and turning up at sites.
I'm sure that we all agree that we don't want RIFs being stolen, single or en masse.
However, it's important to understand that most property theft is opportunistic. Geezers will nick anything. Easy access is more important than the value of the goods. Airsoft RIFs, particularly AEGs, are already a poor haul for a thief. They're relatively bulky, prone to attract all the wrong sorts of attention, and there isn't a big market for them compared to consumer electronics, or even tools.
However, once Pikey McGeezer has broken into a site or a van, he's going to blag whatever's there, just because it's there. He's not going to stop and think "Oh, if these happen to have unique serials numbers, then three transactions down the line, some buyer might be slightly inconvenienced. Nah, best not bother then."
No, the whole lot is going in their van, thence to Freddy the Fence or Daryyyl Dealer, who will do them a favour and give them fifty sovs or a couple of bumps for the lot, take it or leave it. That's if they don't just dump them the canal or a hedgerow once they realise what they've got.
I do not believe that punishing a scant handful of good-faith purchasers well removed from the actual thieves will prevent any theft of airsoft RIFs.