The oysters don't seem to be genetically modified, but bred specifically to produce triploids.
They're fine 
Here is some info i found on google. Does genetically altered mean GMO?
Triploid oysters have been genetically altered so that they are reproductively inactive—virtually sterile. As a result, they attain larger size than normal, or diploid, strains. And because triploids don't undergo the transformations associated with reproduction that make diploid oysters unpalatable during the summer months, they can be marketed all year round. The old adage about not eating oysters in months without an "r" in their name--which probably arose from the fact that diploid oysters become "mushy" during the reproductive phase--doesn't apply to triploids.
Triploids are
genetically altered to have three sets of chromosomes in each cell instead of the normal two. The technique involves interrupting the process of maturation of the oyster egg cell using controlled applications of heat, pressure, or a chemical, so that the egg retains two sets of chromosomes, to which the sperm contributes another for a total of three. Ordinarily, in the process called meiosis, the egg cell ultimately reduces its set of two chromosomes to one before uniting with the sperm chromosome, producing the normal complement of two chromosomes that are found in diploid organisms.