Although the Heavenly Immortal Elixir was primarily made from humans, it was not banned because the minimum standard for the main ingredient was an Immortal. Therefore, those who could concoct this elixir were often not ordinary cultivators and naturally, they weren't prohibited.
After all, who would willingly create trouble for themselves?
However, such matters couldn't be openly addressed, so even Chen Tianzun could not rely on his family's power to arbitrarily capture an ordinary Immortal for alchemy.
Adding to that, he disdained ordinary Immortals, making Tan Shuchang's appearance a case of "sending coals in snowy weather" for him.
Perhaps the forces at play were mutual, or perhaps it was that matters of mutual passion always emerged in the world.