Look at this way. Let's suppose I commit murder and go to jail. My wife and son are perfectly innocent of the crime, so they are not prosecuted, and they don't go to jail. But because of what I have done, they now have to live with the natural consequences of my action: the shame, the guilt, not having dad in the house, etc.
Original sin, as I believe Thomas Aquinas said, is sin only by analogy. It doesn't mean you or I is to blame for what Adam did. But it does mean we live with the effects of it.
As for why the Creator allowed it, well, He had two choices. 1) create men like robots, without free will and constrained to operate on prior programming. 2) give man the power free will, which necessarily gives man the power to put it to evil use as well. Which some do.
It may give you some comfort to know, though, that God only allows evil to happen in order to bring a greater good out of it, like the soldier that gives up his own life defending freedom for someone else.
Yes - - - although that is hard to grasp and understand.