Swear to tell the truth 'so help me God'?

I know that in court if you don't believe in God you are allowed to simply give an "Affirmation" instead of an Oath. But what I want to know is why any Christian would swear an Oath to tell the truth "So help me God"? Where did the idea that this is OK come from? I think the words of Christ were pretty clear, but as far as I know only a few denominations such as Quakers and Mennonites agree with that position (and I certainly don't see eye to eye with them on many key issues). Has there been any justification presented for doing this, or are people just doing what is expected of them from the government? Here is a relevant passage on the subject: Matthew 5:33-37 > 33 “Again, you have heard that it was said to the people long ago, ‘Do not break your oath, but keep the oaths you have made to the Lord.’ 34 But I tell you, Do not swear at all: either by heaven, for it is God’s throne; 35 or by the earth, for it is his footstool; or by Jerusalem, for it is the city of the Great King. 36 And do not swear by your head, for you cannot make even one hair white or black. 37 Simply let your ‘Yes’ be ‘Yes,’ and your ‘No,’ ‘No’; anything beyond this comes from the evil one.