Nostr is open and decentralized. What you described is actually a decent example. Anyone can build anything they want, even a client that won't allow you to export your key or won't allow you to delete your imported key. Now, does that suck and is it a shitty practice? Absolutely. ZBD won't be the only or the last service that does this. As Nostr grows into Other Stuff, this will happen more often. It's up to users to teach people the difference in clients and custody of their keys. This is the same educational argument as Bitcoin. Just because Coinbase holds your keys doesn't mean Bitcoin isn't decentralized. They're just one implementation of it.