Missionary activity is almost an obligation to many fully practicing Christians. I myself was a missionary in Ireland. Yes, I know, hardly a global hotspot.

I didn't bribe anyone either.
I think where this perception comes from is the duel role a missionary takes when he or she goes into a third world country. There is the missionary activity of spreading the Good News, but also in caring for the poor, sick, and homeless. The fact that they go hand in hand leads many to believe they are using aid to promote a religion. In a way they are, as showing kindness to forgotten people is something Christians take seriously as an example from Christ Himself. It is a way of saying "Here is what I believe, and here is what I do because of it".