Ah, I gotcha. In that case I think it makes sense. You can't really subdue someone with your Greataxe...or rather when you do you aren't likely to get any answers out of them afterwards. I assume that this basically means you are making an unarmed strike against the opponent as the only non-lethal method you have, and unless you are a Monk or have the Tavern Brawler Feat (which I don't think has been implemented here) in 5E an unarmed attack is a flat 1 + Str Mod damage.

I'll give it a try to see, but if this is what is happening then I can see a good reason for it from a fluff point of view.