It's interesting to see how other communities deal with the problems of too many unsterilised cat.
Requiring Pet cats to be spayed/neutered
The problem of course, as people have mentioned is that owners with low income (and probably the ones least likely to be able to afford to sterilise their cats) will be the ones who may then abandon their cats. I'm really not sure legislation is the way to go.
In addition, in Singapore as people can't legally 'own' cats in HDB flats, which means there is no way to enforce sterilisation laws.