Restricted to 70mph

I meant to post about this some time ago, but I've not seen it since and didn't trust my memory. However it's just been refreshed by the Royal Mail van in front of me this morning with a sticker on the back informing me that this vehicle is "Restricted to 70mph". How odd as I thought we all were; unless the Royal Mail are delivering the post along long private roads or taking it overseas if it tried to go above 70mph it would be breaking the law.

So why bother informing me? Why should I expect this vehicle to travel faster?