This is not much more than a poorly titled straw man argument. The argument is not that fraud caused the collapse, but that fraud was discovered during and after the collapse, and that fraud should be prosecuted. Ultimately it doesn't matter if the cause of the collapse was legal or not, though it is hard to ignore the fact that it was (at least) exacerbated by fraud.

A more accurate title might be: "Wall Street caused the crash (though not necessarily by illegal means) and was shown, in the process, to have committed numerous acts of fraud which should be prosecuted."