It is a form of "post hoc ergo propter hoc". The benefit was a result, so (pursuit of) it must have been the cause.
It often occurs as "poisoning the well" immediately before a planned "genetic fallacy" which effectively "begs the question".
Look at the situation Mitt Romney was caught in during the 2012 election cycle, where he pointed out that Democrats had created a situation where 47% of the population effectively pays no taxes, and that is not fair to those who do.
He is implying that Democrats purposefully created a culture of dependency to further their own political ambitions. Because it benefitted them, they let the poor make unreasonable demands on the rich. In fact, I doubt there is such a plot. But this is not his main point, his main point is that this makes them untrustworthy and hostile to 'real taxpayers'. But you can only believe this deduction if you accept the conspiracy theory behind the original statement. And we have come full circle -- these folks were not doing the public's (possibly misguided) bidding, they were plotting their own ascent to power.
The Left is just as deep in this boat with attacks on "Big Pharma", "Big Oil", etc. The implication is that these are not mere beneficiaries of a (possibly biased) system we all agreed to, but plotting conspirators. Despite the fact that those making the decisions are obligated by the law -- the contract of fiduciary responsibility to stockholders -- to always act to maximize profit (within the law, reason, and applicable stated charters), we are expected to see their doing so as underhanded gaming.
(The same steps: They won too big => they must have rigged the game [PHEPH]. Just like THEM to do that [poison the well]. Most of what they do is evil, and so is this game rigging [genetic attack]. Therefore hate them for rigging the game [begging the question].)