Obviously there is enforcement. Just saying it's not likely to happen doesn't mean someone won't be stopped/asked, and doesn't have to be prepared for it.

FWIW I was stopped at Hung Hom one time coming in on the through train and was asked what I was carrying, that it needed to go through X-ray, and that happened to be the one and only time I carried something for someone (relative travelling with me) and I didn't really know what it was!