I agree this is a problem - but is it isolated to banking? I can think of at least one non-finance org I've encountered that did much the same: pulled in graduates with big promises and friendly recruiters at milkround conferences, only to slot them into stultifying, dead end support roles where the graduate's skills only withered along with their employability.
I see same problem in academia too, there are a lot of methods to "motivate" grad student I will write few subtle methods which I saw: From default excuse "There are project(scientific, product type, whatever) 'almost done', need few fixes and there will be easy money and you are super skilled, nobody can do this job and etc...", but in reality few fixes mean 80-90% of job and deadlines/delays can take unlimited unpaid time (depends how unlucky you are, but milking won't stop nicely). If got less luck goes like this with signed contracts: There are "incompetent" manager (client side or you side) somewhere in pipeline which don't want go to court and want be nice for everybody (even when case is 100% win, i'm not usa based), but job needs to done and coincidently by you [your team] for free.