The students probably messed up big time by not reporting that the test was publicly available first, but I think we're missing some important facts and only seeing one man's edited version of events here.
1) Ask the professor before the exam if it is okay to use the test bank. He will say, of course not! Why do you have it anyway, it's not made available to students - are you trying to cheat?!
2) Astonished that the test bank was the exam verbatim (though I'm sure few students were), students should have told the professor that many students used the test bank, not realizing he would pull the questions directly from it. Of course, no one would do this directly since students aren't supposed to have the test bank in the first place ...
1.5) Also note that the concept of a "test bank" that is supposedly inaccessible to students is not entirely common knowledge. We don't know how easily accessible and/or clearly warned this information was.
2) I already said that the students were wrong not to mention that they'd seen the test.