Is it common for type faces to be so almost completely similar? SF looks nearly identical to FF DIN to my untrained eye, but even with tiny adjustments, it's bit more than inspired by.
In so many industries we talk about here there would already have been a stream of C&D letters and injunctions would be filed in court.
That is the one part about typography that has always confused me - San Francisco is clearly a variation of FF DIN. Can you buy a 'licenses' to use and make your own variation of a typeface?
From a copyright standpoint in the US, you can copy the glyph outlines of fonts (usually some form of bezier curve or in some cases bitmap) without violating copyright. They are completely uncopyrightable.
Most fonts though (TrueType and OpenType) have a hinting language in them which is code and it is copyrightable since it is computer software. This means you can't just copy DIN font files themselves and distribute them freely without permission.
You can however take the outlines from the font, recreate the hints manually and then distribute it.
That said, there are such things as design patents and of course, trademarks. Design patents aren't used too much, but do protect the design itself. Trademarks of course allow you to protect the name.
In so many industries we talk about here there would already have been a stream of C&D letters and injunctions would be filed in court.