> Does the government require banks to buy long-term treasuries?
Yes, there are a number of banks that have "Primary Dealer" [1] status which confers to them some benefits, but also makes them legally obligated to make some minimum number of winning bids on government treasury auctions. See the section under "Expectations & Requirements" in the link. Probably the most quantifiable requirement is they have to maintain a minimum of 0.25% market share in winning Treasury bids: "Maintain a share of Treasury market making activity of at least 0.25 percent."