It obviously isn't and shouldn't be illegal to run such a service, and I don't agree with the idea that all data analysis should be done on the local device. B

It obviously isn't and shouldn't be illegal to run such a service, and I don't agree with the idea that all data analysis should be done on the local device. But I don't think "your data never leaves the phone" is something that only journalists and people with similar strong privacy requirements would or should value. All other things being equal, I would always prefer an app that keeps my data local, on principle. The difference being a journalist would make is that I would enforce it, and really journalists should know how to do that. They should be disabling network access, both cellular and wifi (which you can do on Android but I'm not sure you can on iOS) for any app they enter sensitive information into.