Hero reset offers some opportunity to replay and grow the hero again whereas adding skill points for gems is a totally unfun way of boosting your heroes, its like paying to cheat up your hero.
From Glu's point of view, a hero reset will always earn Glu more money due to Glu being able to charge a gem cost for the reset and after the reset, the player will pay further 5 gems per level. This is guaranteed money for Glu since the only reason for purchasing a reset is to get the 5 gems bonus skill point. If you wanted to merely reallocate the skill pts, you can already get it from the 1/2 gem per level option.
On top of that, releveling takes time = more gameplay = more ads for Glu.
To that effect, I would advise Glu not to allow skill points purchase but for hero resets so Glu can make more money and continue to develop more awesome content for HoD.