Look at what NCL is doing, from the beginning. Back a few years ago they came out with this free UDP, just pay the gratuities. Every cabin category got the option of picking perks. Back in 2015 even studio cabins got 2. Then the sailaway rates come out, without the perks. Ostensibly these are last minute, heavily discounted rooms. In reality, they're what any other cruise line would call standard priced rooms, repackaged to think they're different. So now you have 2 classes of drinking passengers; those with free UBC (not really free, but w/e) and those without.
Then NCL does two things. First, they widen the range between sailaway and standard (perks included) fare categories. Take a look at just about any cruise on NCL's website. Excepting last minute or otherwise strange voyages, there is a huge spread between standard and sailaway rates. Example: early march sailing on the Getaway, sailaway is $899 pp, standard is $1299pp. NCL is making $800 more from those two customers off the top. This doesn't count the gratuities (lord knows where those actually end up), which are another $277.20 from the same couple. Boom, NCL just generated an extra $1000+ of revenue for giving away a "free" perk.
Now take a look at the other drinking demographic, the sail away non-UBPers. They want to have a drink or two at dinner, sure. A few frozen drinks, perhaps a mixed before dinner beverage, a glass of wine etc. NCL's core demographics are middle class folks who are somewhat price sensitive. So Mr. and Mrs. Sailaway take a peek at the drink menus and just about fall over. They want a drink, but paying for one glass of wine each is going to be almost (or over) $20. Even more for just about any sort of mixed drink. So they may pay for the drinks, or they may not. But they're surrounded by people who have the UBC and are having a fantastic time not giving one care about the price of most drinks. And here Mr. and Mrs. Sailaway are, nursing their one drink because they really really don't want to drop a jackson on another round. As they sit there, they do the math and see that 2 rounds of drinks a day will be more than the "charge" for the UBC (the explicit gratuity), and they remember that they're on a sailaway rate, which to them is a deeply discounted room. So next time, they know they want to do the UBC to save money and have more fun, not counting the near grand they're going to drop to "upgrade" to the higher fare class.
The UBC perk makes money for NCL. It makes more money than people who pay for their drinks individually (or else why do it?). It promotes all day drinking (and the ancillary revenue that comes with this), and increases fun levels for most guests. They don't want people to count their pennies while they drink. By increasing drink prices across the board, they seek to increase the pain of those buying drinks individually, and drive them to fare categories that include perks.
The big thing to look at here is the increasing price of certain mid and high level liquors outside of the UBP zone positions them for this new premium beverage package that includes everything.
tl;dr, NCL making the UBP to seem more valuable than paying for individual drinks, making them more money and positioning their premium package for marketing