I also got the impression that the wedding was cancelled to close to the actual day to get refunds. (40 days before?).
However, they seem to be planning to make the charity dinner an annual event. Not clear if they're planning to cater the next event on a similarly lavish scale (4 course catered meal for 200 at the reception venue). If that's the plan, it's a very generous thing to do, certainly.
Obviously, their money, their choice. Still, I have to wonder whether that's the most helpful way to go about feeding the homeless, assuming that's the goal. That's a big chunk of change to spend on making one evening of homeless person's life better (though I'm sure the good memories last longer).