Something I've pulled from the net:
'The best to rear recommendation comes from the fear of a vehicle loosing control by 'breaking away' in the back end. Imagine a blow-out or an under inflated tyre (due to a slow leak) on the rear and a vehicle cornering at high speed. The risk of the car spinning out of control is very high. Once the car has started to loose control the direction the steering is turned or amount of engine power applied can very quickly worsen the situation with disastrous results.
Having the best tread depth to rear reduces the risk of a puncture and in wet conditions the water can be dispersed more efficiently.
A blow-out or having an under inflated front tyre would be no fun either however, the driver stands a better chance of controlling the vehicle. Also an under inflated front tyre can cause the car to drift one way or the other and the steering characteristics can change, giving the driver some warning of a problem. The same cannot be said for the rear.
Much research has gone into this by the tyre companies on both front and rear wheel drive vehicles.
One of the problems the tyre manufactures face, is getting their front line staff to all sing from the same sheet. There are those in the tyre shops who still recommend best to front'
I think it's a bit of a belt and braces approach and aimed to cover all drivers with varying levels of experience.
It also begs the question, if you had 2 new Ling Long's with 8mm (got to be the worse tyre out there due to price and ebay!) and 2 x 4mm Goodyear Assyms. Should the crap China tyres go on the back!!!! Extreme example I know.