As for open source - it is a developer prerogative.. as much as yours to use the app or not .. but it is NOT a crime - just another type of licence.. In fact pushing the "open source" down everyone's throat is hate ..
Does fountain let you use external NIP-46 signing devices for logging in? Because I'm not giving my nsec to a piece of closed source software.
valid concern .. I am pretty sure they would add absolute anonmity ( in the spirit of #nostr) in upcoming versions .. Meanwhile you may use #fountain with a new nsec minted purely for #fountain .. only if you wish to ... though I highly recommend because I want to support anyone to take the first step towards #nostr .. #goodKarma :-) Even in the current version , it is 100x better than using Spotify ..IMO ..