You did not pre-pay or get charged daily GRATUITIES, you pre-pay or get charged daily the DAILY Service Charge. The only GRATUITIES that you pay are the 20% on dining and drink packages, etc. You are absolutely free to tip anyone on the ship, any amount you want, at any time you want. While NCL specifically states that the service charge supports salary and incentive programs, that does not at all mean it is additive to the base salary as a tip would be. It SUPPORTS salaries meaning it is used to pay the salaries, it does not say it replaces tipping. Unless a person has access to the supporting details of the payroll system for NCL, no one here knows for a fact how the DSC is utilized or distributed. Anyone who says that they do is merely stating opinion and/or tin foil hate conspiracy theories.
The bottom line is that it is none of the passengers' business how the NCL employees are compensated. Further it is quite rude and tacky to inquire of someone how much they make and where it comes from. How much the NCL employees are compensated for the work they do is between them and NCL. As long as they agree to whatever their contract grants them then let them work as they please.