US offered it, US should pay it.
US is able to do it. Because they are able, given the circumstances, they should pay up.
Let's face it, given the current economic situation, $50 million is not that much money in the grand scheme of things. The US is more than able to distribute this money to 9/11 victim's families. This is a worthy cause; it is not meaningless spending for things such as studying the effects of marijuana on pigs.