As for as I know it's law. After the murder of RFK every candidate has the right to have Secret Service protection when it's needed.
Actually, RFK had SS protection, I believe. If memory serves, RFK did not follow the SS's proposed route for leaving the hotel. Or perhaps I am confusing the SS with his private security and/or local law enforcement.
I believe you can only get SS protection if there are credible death threats and/or you are a serious candidate who can win the presidency, either as an independent or one of the prospects for the nomination of a major party. Gingrich has not met this criteria in months. Any other administration would no longer be wasting resources like this.
And Romney is the presumptive nominee, but nobody in the Obama Administration knows how to properly handle resources, so nobody realizes it's probably prudent to pull the plug on Gingrich's SS protection. I am not arguing Gingrich should have never received SS protection. Obviously he was a serious candidate at one time who deserved the protection. I am arguing the SS protection should have been dropped quite a few weeks ago.
As far as I know, RFK didn't have SS protection because he didn't qualify for it. He had local law enforcement protection but that is not the same off course.
As for Gingrich, it's not like Obama decides who gets protection or not. There are probably guidelines to follow and I presume those guidelines where followed. So it's nonsense to blame Obama for this.