It makes sense for it to be free really. If respeccing your character was a big investment then you are punished for not making the right choices while making your character or while levelling up. Players who are most adept at manipulating the economy (making affording the respec easier) are also likely to be the most informed about the game in general (making good build choices) and less likely to need to use it. If getting a respec is significantly challenging then people will go out of their way not to have to do it, diminishing its usefulness.

If respecs still have a cost but its cheap or negligible then you may as well just make it free. You can say "oh why not a balanced cost" but again, what is reasonable will be different for those with system mastery and those learning as they go.

Most importantly, there are so many cool and fun skills and skill interactions in this game, being able to try out fun new moves that you see enemies or your friends use is hella good.