When we have agricultural quarantines here, it usuallly means that the plant cannot be transported outside of the quarantine zone, but not necessarily that it is already infected. When an infected tree is found and reported to the state then there are procedures for removing the tree.
How it worked for bunchy top was a quarantine banning movement of plants from one area to another. This by the way did not work too well. When a banana was identified with bunchy top in a private yard the state sent people over to spray, dig out the entire clump and dispose of the plant material. If bunchy top was on a commercial farm, the state required the farmer to follow certain procedures to get rid of the trees. Only trees with symptoms were removed and sometimes the trees next to them even if they showed no symptoms, but they were not required to eliminate their entire patch.
I don't actually know why the trees at the nursery would be tagged for quarantine but still be allowed to be sold? Does the tag say anything on it?
Happy gardening in Hawaii. Gardens are where people grow.