Without talking politics (this is not intended to discuss the current "how" we came to be getting this money) but talking what the Constitution says... I really almost think this is more like an eminent domain thing. Meaning, we are guaranteed life, liberty, and property against government interference, right? Follow along. So when Gov comes and wants to put a highway (or power lines, or whatever) through your (real estate) property, they have to compensate you for it. Well, labor (your right to work if you so choose) is your property. And they can't just arbitrarily withdraw your ability to find a market for your labor and earn the profits for your labor without compensating you for it. So, they decide, they'll cut us all a check, from our money (taxes), to remove the ability of people to file suit against the government for theft of property without compensation. And then they spin it like they're doing us a favor by giving us our own money back. Or it's also to shut most people up about the 4+ trillion they're giving of our money to big businesses to bail them out (too big to fail, anyone?)
Sure, this theory may be out there, but at least to me, it makes more sense than trying to understand why they're pumping 6+ trillion dollars into an economy where there was already plenty of money in circulation and moving the country much closer to nearly certain default. They certainly aren't sending us money out of the goodness of their hearts nor any real concern for the welfare of their fellow citizens.