Why? Because that's what para A4.2 of RRS says!

If you're talking about a series that is "longer than a regatta" (eg a series with races over several weekends) then the coffee/beer drinkers shouldn't be scored as DNS (which would be starters + 1), they should be scored as DNC (which is entrants + 1). That's designed to cover a situation where some people can't attend every day there's a race. It does mean that the guy who stays ashore doesn't gain an advantage on the guy who is on holiday/away on business/too hung over to get out of bed but lets the person who sails to the starting area and then decides he's too hungover and should be back in bed to gain some advantage on them both.