Even in the post-scarcity United Federation of Planets, there is still a scarcity of time, which cannot simply be replicated. Joseph Sisko only serves so many meals each day and only has so many tables at Sisko's Creole Kitchen in New Orleans. If you want a reservation there, you may have to part with something dear to you in exchange for it. It may not be money — it may be your time or a product of your unique talents — but it's still trade, commerce. There is still scarcity in that world. You will never go hungry, but not everyone gets a starship.