Because overfishing and two fierce diseases have decimated native oysters so that oyster populations are less than 1 percent of their original levels. As a result, Chesapeake Bay waters have become much murkier. Asian oysters are very similar to native ones but resist disease more successfully. Which is exactly what we would want them to do. In this case a non-native species would be filling an ecological niche that has been opened by disease.
Good reasons exist for taking pride in local flora and fauna.
That doesn't mean it isn't valid, but it does mean that ecologists and environmentalists can't simply justify their preference for native species on the basis of economic fiddling that willy-nilly lumps together basically benign alien species along with bad actors. Ecology should not attempt to become a normative science. They should be recognized for what they are and debated on their proper terms, as value judgments that are rooted not in science, but in aesthetics. The fact is that tastes vary. Some people love to look at fields of amber grain and to hear the gentle lowing of cows in a barn.