It's strange people are buying there at all... The only reason I would think of - is to get some altcoins they want, but then again - they can just buy them on an exchange with no problems. They get a lot of publicity, maybe that's the way how they attract new customers and some of them probably does not even care to compare the prices.. Oh, and forgot to mention, they accept credit cards.
I think with GM, Credit Card has a factor as there are lots of people who can not get Bitcoin to start off with so paying with Credit Card is alot easier for them.
Plus you are right, they do a lot of publicity. Do you think you would ever see the owners of Hashnest doing a webinar for example?
Ian