From Article II, Section 2 of the United States Constitution:
“...and he shall have Power to grant Reprieves and Pardons for Offenses against the United States, except in Cases of Impeachment.”
Every court has taken that to mean that the offense must be a federal offense, not just that the offense occurred within the United States.
For example, suppose I live in Maryland. And I am convicted of not paying my state income taxes. The president of the United States cannot pardon me. Only the governor of Maryland can.
Here’s a good article about that:
http://www.legalflip.com/Article.aspx?id=61&pageid=321
Nope. While opinions, even legal ones, are like a certain body part, the Constitution says “these United States”. As opposed to a different country. In the Founding Fathers view the States were to be more powerful than the Federal government. To limit POTUS to Federal crimes, which at the time of the writing of the Constitution were practically nonexistent, would be ridiculous.