Not to mention that bubonic plague is actually endemic in the southwestern United States, including parts of California, Arizona, New Mexico, and Nevada. Somehow, it hasn't killed everyone there yet, even before they had medicines to treat it (and see below on the efficacy of penicillin). Furthermore, the actual deployment of plague by the Japanese against the Chinese, who obviously did not have the same degree of public health measures as the United States, is less than impressive given the scale of this particular operation. Major, sustained efforts using a variety of diseases were necessary to produce five-digit death figures; more damage would probably be done by the inevitable quarantines and public health operations than the actual disease with an operation of the scale described. You might end up with a few hundred deaths and several thousand sickened to various extents if the outbreak gets particularly large.
Oh, and penicillin isn't effective against Yersina pestis, because it attacks (mostly) Gram-positive bacteria, while Y. pestis is Gram-negative.
And again, I have to reiterate that my feeling is not that the United States would not retaliate to the use of biological weapons against the mainland. It obviously would. I just feel that it would not use chemical weapons against the Japanese, but a mix of incendiary weapons, mining of Japanese harbors and seaways, and biological weapons of its own, since those would be far more effective at destroying Japanese capability and will to resist than poison gas. It also might employ chemical weapons tactically in amphibious operations rather than employing purely conventional weapons. If the Japanese hold out long enough, it would employ nuclear weapons, of course, and there would probably be less debate than IOTL about the morality of doing so.
