Moved from the discussion of this answer, which makes the following claim about BSD licences:
The 3BSD licence requires (in clause 1) that your copyright notices be preserved, and that some licence text be reproduced, whenever your code is copied. But it does not require that that licence text actually apply to the copy.
However, to me that seems very unlikely to be true, for the following reasons:
- By requiring the preservation of the specific licensing terms, without explicitly saying "those terms are to be included as a reference to the original author's licence only, but should not be taken as binding", it seems fairly clear that the BSD asserts the same licence applies to copies, with or without modification. A licence notice's language specifically asserts that a work is covered by the given licence. That is a face value reading of the notice's language. Requiring that copies carry the same language without saying it isn't binding necessarily asserts the copies are covered by the same terms, set forth by that language.
- If it were true that copies don't need to be covered by the same licence, then the notice requirement would be trivially sidestepped by simply making a verbatim copy, then stating that it was in fact relicensed under a different license that does not carry the same requirement. Since BSD explicitly does not distinguish between copies "with or without modification", that would be perfectly within the rights given. As it's unlikely that such a reading was intended, I think better evidence is needed to support this claim.
It seems to me that in practice, when BSD-covered derived work is incorporated into code under a different licence, it isn't so much that BSD stops applying to it, as it becomes embedded in the other work, and BSD does not prohibit adding further restrictions and terms (licensing and otherwise) to the resulting work, so long as its requirement of copyright and licence notice is satisfied.