Not sure if that makes sense. By that rational the Cove/Vista cafes could not sell anything, no upcharge drinks in Cabanas/MDR lunches or any bars would be allowed. I don’t recall ever seeing restrictions on those things. Maybe someone else has more details.
I always assumed that the store closings while in port were a decision DCL has made to simplify the shopping experience by avoiding sales tax variations between one port and another, and having passengers upset that they were charged sales tax when they did an in-port purchase in one of the shops that they could have avoided by purchasing while the ship was at sea. I sometimes see complaints about the tax charged on Castaway Cay which is unavoidable when you want to purchase any of the merchandise that is exclusive to the island shops (like the 5K tee shirts that I’ve bought).
I'm not sure it's a cruise line call about closing the shops in port. All lines I've been on that's the standard. Even on embarkation day, the shops onboard are not open until underway.
In addition to any taxes collected on shoreside sales in ports of call, there’s the tourist spending that goes into the local economy, an important consideration for cruise ports, since cruise passengers are (usually) day visitors (with the exception of overnight port stays) who don’t pay for overnight lodging and may not even purchase meals during a port visit, since the ship functions as their floating hotel.
