Park life

It's not that I'm obsessive about language - ok, not that obsessive - but I encounter this sign on almost a daily basis when out with the Handsome Hound. And it's not that "left over" is written thus when most people would reckon that it is a single word ("leftover").

But what is it about rats that I do not understand? I know that some people keep them as pets - indeed, in the house of one friend, I could never enter her playroom when Ron and Reggie were in residence - but as far as I know rats are vermin. Particularly in a public park.

New signs, Southwark?