I think that much of this falls under Christian liberty. There are areas, both in the secular arena and in our own lives, that we will have to evaluate and determine how Scripture and our faith applies.I don't. So much of the biblical principles on which we stand are taken up as political. Given that this country and its government was founded on biblical principles to have the church completely withdraw from government is to abandon our founding.
I would have had no issue with David Platt working towards the mosque being built. There have been several churches allowing displaced Muslim congregations the use of their facilities to worship Allah. While I disagree with both of these, I do understand the need for Christians to live out their faith in all areas of their lives (to include our citizenship).
The problem here is that the IMB has, in my view, exceeded it's commission. I give money to the IMB through my church. In my view, Islam is a dangerous and evil religion in opposition to God. It denies the gospel, blinds millions of men and women, and has killed Christians. It facilitates the spiritual death of millions. I do not see this as much different from petitioning to allow an abortion clinic (as long as it is legal) in one's neighborhood, or allowing a pro-abortion movement to use your church for meetings. Death is death. So I have a problem with my money supporting the freedom for a group of Muslims to build a mosque.