This is a guess. Suppose many Nostr relays charge a fee, requiring users to pay 1 ecash sat for each note they send. At the same time, many mints issue ecash, and these mints are more likely to opt for a model that allows for redemption back into LN sats. Nostr relays would choose to accept ecash from many mints. Both Nostr relays and Ecash mints are service providers, making it unlikely for a monopoly to occur.

But there's a well-known business model where you only allow one-way trade for your tokens. It's very lucrative if you have volume, as people over buy and then can't trade out easily. Think parking meter minutes, in-game currency, mobile phone credits, gift cards, airline miles....