It limits the appearance of culpability. The top brass of the SS can say "oh well we needed everyone and their dog to protect Jill Biden at her event and couldn't spare any more agents" and then hide behind the distinction between a former president and a member of the first family when it comes to the agency's priorities.
Quietly leaving a gap in the security for the shooter to take advantage of is easier than having to position a backup plan, and opens the door to blame local law enforcement.
It limits the appearance of culpability. The top brass of the SS can say "oh well we needed everyone and their dog to protect Jill Biden at her event and couldn't spare any more agents" and then hide behind the distinction between a former president and a member of the first family when it comes to the agency's priorities.
Quietly leaving a gap in the security for the shooter to take advantage of is easier than having to position a backup plan, and opens the door to blame local law enforcement.