But is it plausible to ignore history in such a way? Utilitarianism is committed to "past-blindness". So far as the future is concerned, the past is irrelevant. The universe could be created ex nihilo in its present form, and the future would unfold in the exact same way. But we tend to think there would be moral differences. There is a moral difference between really making a promise, and merely having everyone believe you so promised. Even if the consequences are identical, we think the past matters. You have a greater obligation in the case where you really made a promise.
Suppose you employ a boy to mow your lawn, and after doing the job well he asks for his pay. If you could do more good by giving the money to charity instead, then the utilitarian must conclude that the right thing to do is renege on your promise and give the money to charity. In response to the question, "But is this really absurd?", Kymlicka responds:
Yes, this is absurd. What is absurd here is not necessarily the conclusion, but the fact the boy's having actually performed the job, or that I had actually promised him the money, never enters into the decision as such... The everyday view says that I should repay loans regardless of whether it maximizes utility. The [utilitarian] says that I should repay the loan because it maximizes utility. The boy has no greater claim on me than others, he just is likely to benefit more than they are, and so repayment is the best way to fulfil my utilitarian obligation. (W. Kymlicka, Contemporary Political Philosophy, p.24)
This is also problematic for criminal justice, which we usually see as being heavily dependent upon historical facts. Utilitarianism implies that it is strictly speaking irrelevant whether the accused committed the crime. All that matters is whether finding him guilty would maximize well-being (whether by deterring others, preventing a future crime this so-far-innocent man would commit, or even satisfying the retributive urges of the local community).
More sophisticated 'indirect' accounts, like Rule Utilitarianism, might avoid these conclusions. But, as Kymlicka notes above, they get the right result for the wrong reason. We think that justice requires treating people as they deserve, and this is going to be influenced by facts about the past, regardless of whether they have any future consequences. But utilitarianism leaves no room for the notion of desert. It treats the past as irrelevant. As such, it seems to yield an inadequate conception of justice.