Why they indexed it? Because they found a link to it (live or raw text). Because someone browsed it (with the toolbar). Because it was a referer. Because Google saw the domain being registered. Because someone submitted the URL. Why they didn't index the html instead of the directory tree shown? I see no index/home/default file so it won't load anything.
This is what I cannot understand, Tops The site has all the necessary pages, including the index page and has been live for months. I suppose I should be grateful it is finally in the Google listings. I'm puzzled. Other sites are indexed no problem.
AHA! The year is 2004! (thnx Tops and SiteTutor) I have since purchased and developed the domain name. I also recently wrote to Google asking if perhaps the url had been banned/excluded for some reason due to previous .. whatever? I didn't know. I just knew it was unavaliable in the Google index after many months of waiting. Looks like it has now been reincluded. No doubt the listing will change once the site is properly spidered.
I've seen this on folders/directories that I have that do not have an index page associated with them; mainly images folders and the like. If this is not the case with your site, make sure your robots.txt file is present and that your index page has a robots meta present for G to know it's allowed there. Which site was this done on? That would help in determining the cause maybe ...