Hmmm, I am not sure I agree with that.
It is not usual, but some rabbits and guinea pigs just don't like other animals and prefer to be alone.
Plus it risks people getting into the awful cycle where they really are "done" and don't want any more, say they have 2 elderly rabbits and one dies and they have decided that they don't want any more, if it's the law they have to keep in pairs then they will end up either having to rehome their elderly pet or get another one and be in a cycle that will not end and are stuck with pets they do not want.
I think it is better if it is a strong recommendation rather than a law.