Not wanting to believe everything I read, I went to do a little digging.
Apparently there really is such a law:
Rapid Transit System Regulations
Hawking, begging, touting, etc., prohibited
21. No person, while in or upon the railway premises shall, without the written permission of the Authority or its licensee —
(a) tout or solicit alms, rewards or employment of any description;
(b) sell or offer for sale any article or goods,
or carry on any business; or
(c) display, exhibit or distribute any book or printed, written or pictorial matter or any such samples for the purpose of advertising or publicity.
31(2) No person shall for the purpose of any trade or business transfer any article or goods between the paid area and unpaid area unless the article or goods is taken by a person through a ticket gate.
The parts in red were only added via amendment in 2004.
Then again, if they really meant to enforce such rules, many people would have already run afoul of this daily:
Loitering prohibited
18. No person, not being a passenger or having business in or in connection with the Authority or its licensee or its tenant, shall loiter or remain in or upon any part of the railway premises.
There are many more other funny rules in the Regulations which is good to know and read up on - you never know when you may get hit
. Even things like sitting on the escalator as it is going up (especially when one wishes rest their feet for two stories worth of escalator) is also an offence.