Accomplishes two things.

First, since the Secret Service isn't doing their job ("always a possibility") this lays down an assured-destruction doctrine. This stops Iran from trying it even though it's as easy for them as taking candy from baby.

Second the clandestine state wants to kill Trump then blame Iran so they can attack them - two birds with one stone - but like you say they don't want to risk all-out war because glassing the desert would big time disrupt their plans.