We never thought anyone would, but there was political pressure at the
time from people who didn't (and I see some still don't) understand
the "voluntary" part of the way standards work.
But John - of course any spammer will respectfully honor a "no solicitations"
flag, either delivered by RFC 3865 or the DNS Text record the OP is
requesting... :).
Of course. And they will implement RFC 3514, too.
R's,
John