Under a normally defense ready posture commander in chief,who is not trying to save America for the rest of the world to pillage, this information would not be common knowledge. For operational security it is not revealed to anyone outside the upper command structure, and vessel’s exec, CO, and weps team, for obvious reasons- if it’s known to be carrying W class weapons it’s an opportunity for theft or sabotage or detonation.
There are plenty of things to be critical of Obama for, but this probably isn't one of them. It's no secret that these Tomahawk's have been in storage for sometime. It's also no secret that they are an aging, and maintenance-heavy asset that we've been looking to completely retire (there a difference between storage and retirement) for some time.
If this announcement is helpful in assuaging the concerns of the Japanese people, who are already a little restless about some other items in the SOFA agreements, then it's probably a smart thing to offer to them.