It was going out on a limb - a bit insane, but it was an opening, a chance to take when Remus was almost guaranteed to be taken. Crouch was at the Academy, along with a chunk of the MLE. The Ministry was - relatively - undefended.
That was not, to say, there were not safety measures. Kingsley spun over behind a column between the attacker and them. Watching how the spells hit, Kings figured out where the attack had to be coming from. A smart person wouldn't stay put. So he spent a few hexes out in a few different directions near there.