Presumably they're outside the national limits on the open seas, so there is no law per se.

Is it ethically defensible if it does deter piracy?  How is this different from me going out in a dangerous neighborhood in St. Louis with a concealed weapon and waiting to get mugged so that I can justifiably shoot at people?

I'm not sure what ethics has to do with it.  If there is a bad behavior, then it is justifiable to stop it.  Does it really matter that you're better prepared to handle it?  After all, isn't the pirate's or mugger's behavior presuming that you aren't prepared and that you can be victimized?  This isn't the same thing as when undercover police instigate the act or a crime, where the legalities are certainly questionable.  If the individual(s) in question voluntarily engage in an illegal act which is intended to victimize you, then I see no ethical difficulty in the fact that you were prepared for the encounter.  You didn't set them up, they simply didn't anticipate your reaction.

Unfortuanately I think much of this falls under the same consideration as "one person's terrorist is another's freedom fighter".