There's simply no excuse, and the strangest part is that more people would buy premium themes if they could see what they were getting. Everyone expects to be able to preview, and Microsoft would make more money if they offered previews, yet they don't. So, like I said, idiocy.
Anyway, after 10-15 minutes of failed searching on Google and the Xbox Live Marketplace, I finally ran across a thread on the Xbox.com forums in which someone (not a Microsoft rep, of course) pointed out this site that lets you preview the premium themes. A huge thank you to whoever created the site.
I'm still using the free