crtrav and others may find the article interesting on pg 1 of the city section of SCMP today (15 March). It follows up on this debate and explains how a lawyer believes that clauses on not keeping pets may in many cases not be legally enforceable.
As a side note, it may seem simple to those looking from the outside that someone who has bought a flat where pet ownership is not allowed has made an agreement and should stick with it. However it's not that straightforward. When you buy a flat you may not be focussing on what you want to do there further down the line, and there is so much paperwork you may not even be aware until it's too late that pets are not allowed. Management companies see it as easier just to ban pets outright, rather than have to deal with the few problem owners when problems occur. What organisations like STOP! are trying to do is to enable the well behaved majority not to be penalised by the trouble causing minority, and of course to try to reduce the chances of there being a trouble causing minority at all. This is in everyone's interests - the animal lovers, the non-animal lovers and the animals themselves.