News to me

The job ads in _Eos_include a section called "Student Opportunites"
which lists open graduate assistantships, etc.  The latest issue has
some assistantships with preferences, requirements, whatever one
wants to call them, that I don't recall having encountered before.
One ad for Ph.D. graduate assistantships says, "Preferred applicants
would have a M.S. degree in engineering, hydrologic science, or a
related field with strong analytical and computer programming skills."
That sounds like job ads I've read.  Another says, "Preference will be
given to Ph.D. students with a relevant M.S. degree".  A third says,
"Qualifications include a master's degree or equivalent".  IOW now
they'd prefer not to deal with students fresh out of college, they
want someone who already has a graduate degree.  OK, and I want a
free Lexus, but what is going on?  I hear on one hand that the pool
of available graduate students in the sciences (especially U.S. born)
is shrinking and there are not enough.  On the other hand programs are
apparently so flush with applicants that they can "prefer" those who
already have master's degrees, which suggests that maybe people are
out there with those degrees who can't get jobs and are going back to
school in numbers large enough to allow programs to be picky.  Or
maybe some programs are just getting snooty ("You mean you actually
admit students with only bachelor's degrees to your program?  How
plebeian of you." :-) )  Has anyone out there in academia encountered
this or has similar "preferences" that they expect actually to be
fulfilled by their recuits?

Regards,
Russell