Considering the wide variety of pricing that occurs, If they did add it on, I doubt anyone would notice..
Personally, I do not like being told what I should tipo, and adding it to my onboard account automatically is the height of presumptiousness. How much I tip and to whom is my business and not the cruise lines.
If I want to put it on my account, then I should have the choice, I do not like like having it added on for my convenience. Too many times things that are done "For my convenience" turn out to be not for mine but the person doing the charging in the first place.