For those who think this is somehow a Delta specific IT fail, it's an artifact of the design of pretty much every modern web site: Your web browser acts like a client and authenticates to a server
as you (thus the part where you had to login to see this data), then your web browser customizes your browsing experience according to the data pulled from the server. (See, eg,
AJAX.)
GMail, for example, uses this exact model (assuming you use the web interface), only the data it's pulling is your email instead of your home value. Of course, Google has access to lots of data about you, too, that's how they target ads to you. You just haven't discovered the same "breach" (that isn't really a breach) yet.
If you want to get upset at Delta over this, then you need to get upset with almost every other major corporation in the US at the same time. (If you want to get upset at Delta over your CustValSeg being 0, that's a different issue.)