Not sure about your math, here. The beverage package on NCL is $79 pp
per day. So on a 7 day cruise it would cost $553 pp. They add on an 18% service charge to make the total cost roughly $653 pp. If you get the "free" beverage package as one of your perks, they charge you the 18% service fee based on what the package would have cost had you bought it separately as part of your cruise fare. Honestly, I have zero problem with that. If you have a coupon for a "free" meal at a restaurant on land, you still tip the servers (or at least you should). I don't see this as any different, but reading CC's NCL board I see that others think it's some type of bait and switch. When NCL first started offering the drink package as a free perk at booking, they did not charge the service fee up front and left it at guest's discretion to tip the bar tenders. I'm guessing that didn't work out so well for the NCL bar staff.
They then began charging the service charge up front and you'd have thought NCL came to people's home and killed their dog, reading some of the reactions.
Now, if you aren't much of a drinker it might not make sense to get the free beverage package as your perk. It very well might save you money to just buy your drinks individually on board. But if you think you will spend more than $99 pp (18% service charge added on to your cruise fare for a 7 night cruise) on individual drinks, the perk saves you money. If it doesn't, simply pick one of their other perks. They have several options.