As UK citiziens, they are impacted by decisions that affect UK citizens, even citizens living abroad. Most obviously Brexit, but it’s not difficult to see that there might be other decisions, e.g., around tax and benefits, that would affect them. I know various people with up to three different citizenships and they can vote in all three of those countries, for this exact reason.

Obviously the real reason the Tories have allowed this is totally cynical, but that doesn’t mean there isn’t a non-cynical argument for it.