At this point, with the investigation back at where they started, what would Beth be jeopardizing by telling the media some more of what she knows? Perhaps if she was to reveal a bit more, such as why she thinks Natalee was at the vdS home at 3:15am, it would put some pressure back on the vdS's.
That's what I'm wondering. Unless the FBI has told her to keep mum on specifics.