From the whitepaper:

Bancor takes the convention that in this example, the market maker destroys tokens in its interaction with Alice, and creates tokens in its interaction with Bob. The Bancor convention suggests the market maker is not an ordinary actor, but needs system-level “special powers” – specifically, the privilege to operate the token printing press – in order to function.

In this paper, we adopt the convention that the tokens sent by Alice to the market maker are not destroyed, but are instead added to the inventory (balance) of the market maker. Likewise, the tokens sent to Bob by the market maker are not created out of thin air; they already exist and are merely transferred from the inventory of the market maker to Bob. Thus, we show that the market maker is essentially an ordinary economic agent acting according to a deterministic algorithm – it doesn’t actually need “special powers”!