I have no idea why the Church signed up for that, so I can't speak to that.And by the Mormon Church's choice, they are making it an actual prosecutable crime against the state of Utah to be caught with a gun in their church in that state. That's a pretty high-handed "position" to have.
The position itself is flawed. God has no need of state laws to protect Him. And God makes use of armed followers to protect each other. No church is immune from crazy people coming inside, ignoring a sign or a posting on a state website.
However, I can say that when I was younger, in the Boy Scouts, we had meetings in the meetinghouse. At one time, we went through the NRA firearms safety course. When it came time to handle and use actual guns, the meetings were moved to the home of one of the members.