This Presidential Pardon bit - the United States Constitution, Article II, Section 2, states that the President "shall have power to grant reprieves and pardons for offenses against the United States, except in cases of impeachment."

So how does it comes that he seems allowed to pardon people for things that aren't offences against the United States? For example, in this case (allegedly) offences against prisoners, who might not even be US citizens. Or in another context, alleged responsibility for war crimes against people who were definitely not US citizens.