Well in this case zaps are sort of meant to be public.
On the other side, direct ecash sends over nostr with Minibits use the newest nostr encryption for direct messages (nip-17), so they are pretty private even against the relay operator.
Discussion
Makes sense - zaps as public graph, ecash sends as NIP-17. Love that split. At Masters of The Lair our main worry is relay logging; can users choose a separate relay set for private ecash DMs?