Or if they receive an UNALTERED copy from you! Because if you change the licence (which you're not allowed to do) it's not an unaltered copy :-)Please don't not mix licenses and license grants:-) Let's consider it in more details: suppose I distribute your source code non-altered or non-creatively altered (so I don't have any copyright in this work) with GPLv3 attached and all references to other licenses (whether GPLv2 or BSD) stripped. AFAICS it's clearly permitted under clauses 4 and/or 5 of GPLv3.
In which case, you HAVE altered my work. You've removed part of it, namely the licence grant.
Oh - and that probably is a very definite copyright violation :-) I didn't grant you a licence to do that, I granted you a licence to alter the program :-)
And as someone else in this thread said, if they get one copy via one route that is GPL, and another via another route that is BSD, they think they can apply either licence to either copy. This is a very vague area.
But as far as I am concerned, legal niceties aside, if I dual-licence my work (such as, let's say, making it GPL v2+), if you strip off the v2 and change it to v3+ you are misrepresenting me to my users, and you are stripping my users of the rights I granted them. Doesn't the GPL 2 itself say "you mustn't impose further restrictions"? What is removing the option to use v2, if not an unpermitted "further restriction"? While this may be a legal grey area, it isn't a grey moral area - it's just unacceptable.
Cheers, Wol -- Anthony W. Youngman - anthony@thewolery.demon.co.uk