Why? Because IE was a default browser, all the OS have a default browser. Microsoft didn't prohibited anybody from installing another browser.
If we consider that a monopoly and something punishable, then Android + Chrome is another monopoly. In EU, Android has over 61% market share [1]. And every Android comes with its default browser. And as your parent said, I don't see any ballot screen telling me to choose an alternative browser, but Google doesn't keep you from installing other browsers. Is the exact same situation.
[1] http://www.nasdaq.com/article/apple-raises-market-share-to-5...