The line output is mono, so this is expected (and desirable!). You'll find the same behavior in any piece of electronic musical equipment that doesn't have stereo output (or separate left/right outputs, which is what professional equipment with stereo outputs usually has).
Why would you want to connect headphones or speakers there anyway? That's what the headphone output is for. The headphone output is still not really stereo, but it has a tip/ring/sleeve connector that sends the mono signal to both channels.