Why not just add $20 pp ? Because NCL makes more money this way. Most passengers don't even know about the soda cards. They only sell them the first day or so on most ships. Not everyone spends time on the boards in preparation for a cruise.
Many folks end up spending 2 and 3 times the cost of a soda card and still enjoy their cruise. I thank them it keeps the cost of my cruise down. They do as much as those that buy the prints at the art auction or shop in the stores that pay a promotional fee to NCL..