The Pope visited there last year. A number of Presidents have been there. For example, President Johnson's daughter was married there in 1966. In 2008, President Bush attended the funeral of his former press secretary, Tony Snow, at the church. They are very accustomed to major security events.

In contrast, the President and VP both attended Clementa Pinckney's funeral's in Charleston, held in a sports arena twice the size of the church and with admission open to all members of the public (in contrast to the Scalia funeral which is invitation only).

If there is a good reason for Obama not to attend I haven't heard it. Yes, it's not required. But given the current political debate in which the repubs are back on their heels for overreaching in their partisanship, the right move is to go above and beyond and disarm the critics rather than allow them (and the media) to change the conversation, as they have.