They actually do license out their content. At one time I was developing a piece of software to catalog my music collection and was very interested in accessing the data directly. However, it looks like it is more geared towards larger clients (i.e. amazon) and I have a feeling the cost would be in the thousands, if not tens of thousands, per year.
Since I only want to use it for a small searchable catalog for personal use I ended up writing something to connect to the site and extract the data into my own database. I guess this wouldn't work any longer if they switch to a subscription service and I would really miss it; I find myself using the AMG site for reference at least 5-10 times a day and I'd hate to go back to relying on sites like amazon or cddb.com again.
For those wondering, the database is about 250MB compressed, which would fit on a cdr or any hard drive made in the past 6-7 years. Decompressed it is about 850MB to a little over a gig, depending on how you read it.
link: http://www.allmediaguide.com/tech.html