My best guess is that Hilton purchased your email address along with your name to send the promo to you, not realizing that you already have a HHonors account (thus basically wasting a purchased email since you would have gotten it at your normal email address anyway).

Where they purchased the email address is anyone's guess. Possibly US Air sold, or "rented" (such a horrible phrase that some companies use), your email address. Possibly the geneology sites did. Possibly it was a guess, which email marketing companies certainly do from time to time. This last option is really only valid if your email address is very similar to your actual name.