I don't get why. They are banks that have LN (or other channels) in between them (if I get it right).
I use mint A, you use mint B.
You send zap = You tell mint B to send BTC (LN or other) to mint A and mint out that amount to me.
You're just outsourcing the LN node + channeling to a more common level (which should ideally be quite small and private imo).