God forbid RPGs make your choices permanent. And how exactly would you even go about implementing such a feature? Where's the line between switching a misplaced perk or two around and re-perking your entire character from a pure Warrior to a pure Mage? How would you implement the feature, exactly? I really don't see it. If you could switch perks around, you'd be able to by-pass the entire building process of your character. A level 81 heavy armor, sword and board warrior could become a pure mage just like that. If they only allowed you one chance to change 1-2 perks, then people would complain that even that isn't enough. They're better off just not implementing it. If they add that, soon people will start asking for features to reset your stats as well, "because I chose too much stamina and not enough magicka" The whole idea of the system is to choose carefully and if you regret it in the future, well then that's too bad. This isn't WoW where you can just respec whenever you want to and I'd rather they focus their time elsewhere rather than implementing stuff like this.
While I understand your question about one or two perks versus an entire character 'do over', I have a question for you. My main character, level 81 light armor sneaky thief archer type, was 'specced' with the knowledge that there were no dragon bone weapons. I made the perk decisions very carefully and cautiously and, although I regret one or two, have been completely happy with the choices I made...until the DLC came out. Because glass armor could be smithed as high, or higher, as dragon scale, and I never planned on being a heavy armor type, I didn't choose dragon armor under the smithing tree. Now I have to rely on finding dragon bone weapons instead of being able to smith them. I never intentionally turned down the ability to smith dragon bone weapons, because that option wasn't available. Shouldn't I have the option to smith them with my main character instead of having to start all over?