Think of the replacements for the plastic items that will go into use on the ships, and then consider that cruise ships already often have problems with communicable diseases among the passengers.
And, the ban ignores the latest science on the breakdown in plastics by natural forces, both on land and in the seas. That breakdown is more common and much faster than earlier claims said it was.
Sun, the waves, and the sun+water over time tear the plastic apart, turning most of it into tiny pieces that microbes devour. Microbes eventually consume the plastic in the ground, taking sometimes only decades or centuries, and not the 1,000+ years previously thought to be the case. The microbes turn the plastic into what they can use of it and release carbon dioxide and carbon back to the earth.
The alternatives are glass, metal, and paper...all of which require more fossil fuels to make than plastics, plus the additional weight.