Why are new users automatically subscribed to /r/atheism?

Throwaway here.

I love reddit, but I just wanted to ask why /r/atheism posts show up on the front page by default and why new users are automatically subscribed to the subreddit.

I encouraged my friend to become a redditor, but she was turned off on her first visit due to the amount of atheist posts indirectly bashing on her religion. I explained to her about unsubscribing but she didn't really seem to inclined on coming back. I don't really blame her; first impressions sometimes really matter.

I have nothing against atheism and I understand that the atheism subreddit is one of the most popular, but shouldn't the default site have less controversial subreddits (such as r/atheism) and more new-user friendly subreddits (like r/aww)