Bush was campaigning on welfare money. By "on," I don't mean he was advocating increasing it (yeah, right), or even decreasing it. He was spending it! Because he babbled a little bit about welfare reform while campaigning for candidates trying to eliminate it, Bush decided he could get welfare to help pay for his trip:

On stealing from the poor to give to the rich: In an act of reverse Robin Hood effrontery, the president helped defray some of the cost of his nonstop campaigning with an accounting trick that allowed him to dip into the coffers of the Office of Family Assistance by piggy-backing campaign appearances onto trips ostensibly made to talk about welfare reform. That's right, money meant to assist poor families was used to help elect politicians who believe that, even with all the problems facing this country, cutting taxes for the rich should be job one. -- from Arianna Huffington.