On the other hand, if you have ever been arrested, even if not accused of anything, and you want to apply for citizenship or a green card in the US, then the burden is placed on you to get proof that you have never been accused of anything [1]. If you fail to provide such evidence (as can happen if you were arrested outside of the US and the police in that country aren't interested in cooperating), then the default presumption is one of guilt rather than innocence.

Edit: And in case it wasn't clear, failure to obtain a green card in the US can make it more difficult to find employment, as not all companies are eager to navigate immigration procedures on the behalf of their employees.

[1]: http://www.uscis.gov/sites/default/files/files/form/i-485ins... (page 3)