Self-explanatory... It doesn't matter whether it's the B-Button set for the controller or the B-Button set for the keyboard.
Pressing the B-Button during name registrations does absolutely nothing!
I have to register at least one character immediately after I select the 'Register Your Name' Option, thus forcing me to delete that file after wards.
The B-Button still cancels the copy and delete options, however.
I can confirm the explained behavior on build 887, I can not be bothered confirming the behavior on earlier builds though >.>" sorry.
This is intentional and has been this way for a while, I thought. The reason is that some people have their A and B buttons set to letter keys (like z and x) instead of Ctrl and Alt. So, when they try to type in a name like Zelda, it cancels. Esc is used to cancel out of name entry mode.
Sounds logical, thanks for the heads up!