They own a bond which pays 6% per year.
I don't see how that differs except the Fed is failing to mention the profit of member banks.
Why would they? If you need to know the earnings, many excellent annual reports are available.
We're talking about the shares that member banks buy from District banks; let's look at the law.
Banks buy shares at "$100 each" (from FRA Section 5). Now, when most people buy stock shares in a private company they may want to control or own the company and sometimes it's for capital gains or dividends. FRA Section 5 specifically forbids these goals by flat out saying that "Shares of the capital stock of Federal reserve banks owned by member banks shall not be transferred or hypothecated" and the fact that Congress even retains control of management.
The only reason banks buy shares (re- FRA Section 4) is so they can "avail themselves of the advantages of this Act." OK, so it's not much compared to what can be had with other kinds of shares, but there are a lot of banks happy enough with the deal.