The answer
Look at who was doing the checking, and who wasn't, in both cases. Look at the resources that were devoted to both. Where did you read about Frost's background? In the "New York Times," or in a blog? Was the information gathered by professional reporters on expense accounts with travel budgets and camera trucks, or by amateurs looking through public sources?
If Frost had been in a Republican speech, the mainstream media would have flayed him alive. This isn't about principles or privacy, it's about politics.
Does it bother you that the organizations that deliver news to Americans have this kind of political bias?