Because they are pocketing the difference by arbitraging between the government (who issue the treasuries) and the Fed (who is the ultimate buyer) since the Fed cannot buy treasuries directly from the government. It's just a complicated way for the government to print money and hand it out and in this case banks are able to act as the middleman and earn money on the spread.
It's a disgraceful system that is extremely morally questionable.
It's a disgraceful system that is extremely morally questionable.