China banned U.S. poultry imports Tuesday to ease growing fears about bird flu, announcing in an emergency notice that any American fowl that have already arrived at its ports would be returned or destroyed . . . In December, China banned beef products from the United States after a case of mad cow disease was reported there. That prohibition has not been rescinded.I'm no expert on the issue. Perhaps someone could tell me how this doesn't necessarily escalate to a sort of back-door protectionism.