I actually somewhat understand what VLE is saying. There is no doubt that many people he, volunteer, or give charity for reasons other than pure altruism. It certainly doesn't change the effect of the act, but if you are acting to fulfill some religious or spiritual doctrine, the intent should be there as well.
I have read the argument that you should claim the donation and give the money saved to the church, thereby increasing the donation. How many people actually do this? I am sure that it's not many.
It kind of reminds me of these large companies that do charity work and then, list all of their charity work on their website or include information about their "community citizenship" in their brochures. Did they really act out of a sense of charity or out of an obligation to appear charitable?