Dual licensing gives every recipient both licenses from the original author.
Really? Which license or law gives you the right to change the licensing
terms on code you didn't write? At least in the United States, you cannot
change the licensing terms on someone else's code absent a written
re-licensing agreement.
Every recipient of dual-licensed creative works or elements of works can
obtain rights from either license. You only need comply with the
requirements of one of the licenses to obtain rights from it. If you
redistribute the code, every recipient gets the same rights you got from the
original authors of every creative element you redistribute.
The distributor can choose by which license he obtains the right to
distribute, but this has no effect on the license his recipients get! (This
should be common sense. If you buy a music CD from Amazon, do you think how
Amazon got the right to sell you the CD should have an effect on what you
can do with the CD?)
You can remove the BSD license notice, because the GPL gives you that right
(it only requires you to retain notices that refer to "this License", the
GPL, not other licenses). But removing a license *notice* does not change
the license. (That should be common sense too.)
If you modify the code, of course, they only get the rights you choose to
give them to anything you added (assuming it is sufficiently creative to
warrant copyright protection). Nobody can grant other people rights to your
code that you don't want to grant. But you cannot add or subtract licensing
terms to code you did not write (at least, neither the GPL nor the BSD
license give you this right). The license grant is direct from the original
author to the ultimate recipient.
The GPL makes this explicit in section 6, "Each time you redistribute the
Program (or any work based on the Program), the recipient automatically
receives a license from the
original licensor to copy, distribute or modify the Program subject to these
terms and conditions. ...
