I suppose for a lot of people they probably wouldn't have a drink if they were driving the next day but if someone if travelling on a ferry overnight and it's for their holidays they may be inclined to have a drink at night on board.
Two issues, the limit in mainland Europe is less than England and Wales but the same as Scotland and although this was mentioned in the news in December still people don't know the limit on the mainland vs England and Wales. Also even if your not drinking the fact every driver is being stopped and checked will be adding on time leaving the port. Not so bad if your at the front, but if your at the back it may add up to a considerable wait.