Not that I remotely want to defend reddit, but from a development standpoint it's much easier to maintain and secure a single login workflow. Whatever nonsense the new/old font ends require, it's probably much easier to make that work with a single unified token than it is to maintain both separately.
As long as the login remembers which frontend it came from, I wouldn't be too up in arms here. If it dumps you out to www/new.reddit then that's completely fucked and there's no excuse.