Why do we have to commit to using our Hero Abilities once clicked?

[Trevenaunt]

I'm using the Call of Harmony ability for my Scoia 'tael deck. Why is it that when we click on the ability that we MUST commit to it? Why cant we cancel it? I wanted to use the ability to summon Munro Bruys but then realized that I had the play order messed up so I wouldn't be able to activate his deploy effect. I wanted to just undo it, cast Zoltan's Company, THEN cast my hero ability. Why are we so limited? I gained absolutely no advantage by clicking my hero ability so what is the reasoning behind forcing me to use it without the ability to undo it?

 

[Iuliandrei]

With most leaders, those who have targeted abilities, you can cancel it, but some leaders don't have this option. It happened to me once aswell to misclick on my leader and couldn't cancel the ability.