Legally, probably not. They aren't demanding that you not redistribute, just asking. And asking politely. They are banking on the good nature of people. Good for them! And us.
Exactly. The Condor Project did this for many years and, miraculously, every one of the hundreds of users to whom we gave the source respected the request and the source never leaked. (It is now fully open.)
I can't speak for them, but in our case we had major users who required open-source (mostly European govt. projects).
All our developers wanted to just post the source online and be done with it, but our boss had security concerns he wanted us to address first -- so in the meantime we gave people who needed it GPL'ed copies, and asked them politely not to redistribute until we were ready.