They went because it was NOT an unknown ship, it was an Earth
registered ship, and they had done considerable scans (as noted in
dialogue) prior to entering the ship. If YOU were the captain of a
modern carrier, and you found a ship from 100 years floating past,
wouldn't YOU want to investigate it? There was absolutely NO clear and
present danger. Further, they were not, in fact, attacked by anything
in the ship; the thing would have gotten out whoever had opened the
cryogenic freezer. So what difference would it have made to send in
some "expendable lackies" when nothing was at risk anyway?
This is, frankly, Picard-think. When TNG went on the air, everybody
yelled and carried on about how Kirk wouldn't let someone else go down
to the planet instead of him; after a few years, suddenly it became SOP
that the captain NEVER goes into any situation with possible jeapordy.
You judge cases on an individual basis. There was no reason for
them NOT to go into the ship, every reason FOR them to go into it, and
nothing whatsoever happened once they WERE on board the ship to show that
their safety assessment was incorrect.