I don't think it is a public relations problem either (nor should it be).
The FBI is an employer (much like any other employer). Employers give their employees days off for all sorts of reasons. As far as a snow day, there are plenty of factors involved in this decision. For example, if a snow emergency happens here, at times the mayor will request all companies to keep their employees home... this makes it easier for emergency vehicles and cleanup as well.
My company has a list of "essential personnel"... people who know their responsibility to keep things running. I assume that people working on essential cases at FBI did the same.
This is a non-story.