The problem is that crime comitted in a car isn't seen by many as a crime.
Kill someone with a knife and it's murder. Kill them with a car and you get a slap on the wrist.
People moan a little but simply don't consider anything done on a motor vehicle to be wrong. Look at the numbers moaning about being caught for speeding. THEY are a good driver, better than everyone else. THEY shouldn't be hauled over the coals because they drive too fast, don't pay attention, use thier phone or any one of a hundred other infringements. A bit of drinking is fine, THEY are able to handle it.
And as for celebs, well most people are so in awe of them, they can't see why they should face any justice at all. Plenty of people were happy to defend Clarkson thumping an employee, but I wonder if they would have been so happy if it were their child being belted for not serving up s steak dinner?
Combine driving with a celeb and I bet there will be people lining up to donate to a fund to pay his fine!