The problem is that this is difficult, especially for younger people.
I've had various online aliases over the years, most of which have been abandoned because they no longer reflect me at all.
Let's say you were 16-18 and naive and got roped into some extreme political or religious cult group (this didn't happen to me btw). You might post all kinds of things and at the time you would probably be totally convinced that these were things you would be happy to share forever.
Basically, we need to tell kids "don't post anything online under your real name unless you would be happy to tattoo it onto your forehead"