Yes, that is impossible. Someone can claim to be an atheist (whether they are or not) to spite their parents, but they can't change their internal dialogue to spite anybody
Try this little experiment: sometime when you're feeling rotten, force yourself to smile. Then give a full belly laugh. You'll feel better afterwards. If you pretend to be something long enough you will turn into that thing.
Or, as Steppenwolf said "wearing masks that turn to skin, hiding what you could've been"
If that doesn't satisfy you, try these on for size:
Suppose the parents are abusive, and the child comes to associate the abuse with religion. Said child then declares religion to be false because of the conflict between his/her parents actions and religions preaching about love.
Or alternately, a child is lovingly raised in a strictly atheistic household, but lives next door to a crazed "hellfire and brimstone" type who insists that all atheists will burn for eternity. Hearing someone claim that their loving parents will be harshly punished would doubtlessly cause the child to doubt the validity of the religious person's claims, and thereafter would probably be dubious towards any religion.
Neither of these are logical decisions, but emotional ones.
or because they simply don't want to accept the possibility of consequences for some of their actions?
That sounds like lunacy. What does that have to do with being an atheist or theist?
Suppose I deeply enjoy eating ice cream (okay, so maybe I really do). I've been eating it all my life and intend to keep eating ice cream. Then a religion comes along saying that eating ice cream is a mortal sin, and I will be punished for it. I'll be even less likely to accept things said by that religion than I normally would because I don't
want it to be true.
Again, it's an emotional decision, not a logical one.
If they hadn't put much thought into it then they don't hold any position. Right?
I've met many people of a wide variety of beliefs, including atheism & agnosticism, who held their beliefs simply because of that's what their peer group frequently held. Consider:
http://xkcd.com/185/