Maybe it should do it by default in the background the first time you log in with a new npub and occasionally after that by polling randomly. Kind:0 is pretty important to not lose
Guessing a more probable reason could be that the app has default relay(s), then the user updates their relays and removes those, then the app doesn't rebroadcast. Does NN rebroadcast kind-0 when updating relays?
You can always fake zaps by just paying someone to zap you. Proving that someone really sent you money is not solving anything. What are you even solving by putting it on chain?
I can spin up npubs and zap myself on chain, paying only the TX fee. So you still have to WoT the zaps, same as you are with private LN payments.
Specifically the combo of onchain zaps and amber removal from 2 top clients this week is some insanely bad timing. We've seen many nostr users accidentally download fake apks from the zapstore that show u pas an update to their app. Now theres a direct financial incentive to peddle all kinds of fake apks to steal nsecs and drain onchain wallets.