Actually, I think the issue is you are taking the default for the MOVE
DRMEDIA command which is SOURCE=DBFULL.  If you specify SOURCE=DBS, it will
get the DBSNAPSHOT tapes.

I use DBNONE and then issue a specific MOVE DRMEDIA command for each
DBSNAPSHOT volume that I want to eject.  The reason is we run a DBFULL
onsite and a DBSnapshot every day, plus 2 offisite DBSnaphots.  That way I
am fairly certain that I have a DBSnapshot that I can restore from in a
disaster.