I have to take my mother to the supermarket a couple of times a month as my father no longer drives - she is elderly and I am past my prime - she loves the zed and we always use the parent and child bays for damage limitation reasons- I once had an altercation with a mother in a Black Sharron with 2 very ugly children - she got quite personal in the end after being very polite and very "British" I ended up telling her to get a life and to feck off - priceless LOL
It is simply because they are just twats, trying to be funny...that is the answer.