What makes these mechanics desirable? Is it just because you like the feeling of killing things in one shot? Contrast that with how frustrating it is when you are killed so fast that you have no chance to react. You literally had no input on your own death. Your own death from a random grenade or a single shot at distance is in no way under your control. The ammount of skill you have has no impact over you surviving in these situations. Do you like having no control over your life/death in these situations?
Wouldn't you prefer to have the ability to fight back, and through skill defeat your opponent, rather than be forced to die without a chance to exert your own ability?
You most certainly do have control over these things, just not in the same way as situations where you can escape after the first shot. HS kills promote a different style of play, being more attentive to your surroundings, and a little more cautious with how you proceed. The shooter also has to be careful in these situations because the shot gives away his position. I don't need OHKs, but I still hope that an appropriately placed HS still causes incapacitation with a strong enough weapon.
I understand the issue with grenades, as the random toss has become extremely popular in most games due to not having the need to conserve ammunition/grenades. People end up throwing them without a care in the world and if they get a kill they get a kill.