Aerospace has stated it well.
On a 7-night cruise like yours, you are required to buy it for the entire trip - no option on that one. I do think it's dumb that they give the cost per day when you have to buy it for all 7 days - would make more sense to simply provide the cost for the trip!
But was stated, on the longer cruises, they will usually sell them not just on the first day as they do on the shorter cruises, but also for the first few days (at a per day reduced rate).
I agree about the 'drinking that much each day'. I can usually do my fair share of drinking, but when you figure the cost for the entire trip, I would have a hard time making it worth it. Definitely would encourage me to do ALOT of drinking!