Why write "Polite Notice" on a sign?

I was parking my car this morning, and saw a sign attached to someone's front gate...

Polite Notice:

Do not park in front of these gates at any time.

...and I wondered about why the author decided to include the words "Polite Notice". Why not just say "please" instead?

To me, it gives the sign an odd feel. As if the author is reviewing his own sign, and saying it is polite, when clearly it is quite rude and abrupt.

This is a very common practice (in the UK, at least), but I find it very odd. What is the logic behind using this wording?