I think it's funny how pretty much every westerner who is new to Futaba/2channel style boards is chosing a nickname by default. This can be observed pretty well at the increased posting rate on the iitran discussion board ever since they started doing the Emma anime fansubs.
There are three possible reasons I can come up with why this is the case:

• They don't even know about the option to stay Anonymous on these kind of boards.
• They are used to using names and being identifiable by those names (while probably none know about how ID codes nor tripcodes work).
• They don't want to be Anonymous - the whole concept of anonymous posting is not alien to them but rather undesirable. They want the attention, the recognition, the identification.