I know this is a very old post, but I had basically the same question, "Why is ASPNETDB.mdf being created?". Since my site used a SQL database, ASPNETDB.mdf had not previously been present. But then suddenly it appeared and my entire site crashed at the hosting location. It was a little insidious because everything worked fine in the development environment.
In my case I wanted to get a list of user roles and I had added < roleManager enabled="true" /> in web.config. I didn't connect at the time that this would create ASPNETDB.mdf. This SO post The Role Manager feature has not been enabled has some good discussion regarding this topic.
But the point is that when I enabled the role manager, the MVC framework automatically created ASPNETDB.mdf, causing my problems at the hosting location. I found a different way to get the roles without activating the default role manager and my immediate problem was resolved.
I just wanted to add this post in the event someone else was surprised by the sudden appearance of ASPNETDB.mdf. This is another way it can show up, which is related to the op's question.