I would have more of a problem with this if it was a simple fork with a few variables modified to enable features rather than a complete rewrite with a compatible API. None of the paid features are technological breakthroughs (e.g. one just checks HIBP to see if passwords have been leaked) so, imo, it's really the re-implementor that did all the work.

I do feel a little bad using the client applications for free given their business model revolves charging for the backend only.