It is part of Capitalist system. State central banks are State Capitalism, while Private Central Banks (US FED & I think all central banks in western nations) are Capitalism.
The "part of" is the central point, monetary policy may be part of, but not the whole of capitalism, and any system will have a monetary policy of sorts. Even Bitcoin have ContSupply.
Don't get me wrong, not all Capitalists are shit people, not all exploit their workers (some more some less, some care of their workers very well), point is SYSTEM allows bad behavior
Of course its not the whole Capitalist ecosystem but its the top of its pyramid structure.
