It has to do with legislation and approved sources. The treasury has always been authorized to mint coins and commemoratives by specific weight, diameter, and fineness. The ASE was authorized under the Liberty Coin Act (1985) as an amendment, but the driving force goes back further. As a nation, we had a strategic stockpile of silver for National Defense. In the early 80's, Reagan had wanted to sell off some of the stockpile to balance the budget. But of course, with the low silver market price (post-Hunt Brothers), doing so would effect silver mining jobs and the effort was blocked by Idaho senators in the interest of the silver mining industry. The ASE authorization was later slipped into the Liberty Coin Act as an amendment that specified the source be derived from the strategic stockpile. The stockpile served as the source for ASE until 2002 when it was depleted and an extension to reauthorize the ASE authorized the source to be from private mining interests.