Apart from $, the point is to share the gospel using such orgs, for those orgs that do evangelize cannot get the access these secular ones do. Thus certain Christians are as undercover agents, not so simply giving time to these groups humanitarian works, but using such as cover to share the Truth of the Lord Jesus. Brave souls of faith in my eyes.
I understand “missionaries” who go under the guise of doctors, teachers, etc. That is understandable and commendable. But each year at our church we stuff boxes for children overseas. We donate time and resources for this activity (to name but one), yet we are not allowed to put any religious material in the box or they will confiscate it (the organization). I’m not sure what is the point.