I hate to state the obvious, but when solo diving, if anything happens, you are the one who took the risk, so you are the one responsible.
Why is it so terrible to allow someone to take responsibility for their own actions, diving or otherwise?
When we were children, we needed our parents to hold our hands to cross a busy street. Now that we are adults, we can assess the street and cross it ourselves and most folks are fine with that.
I think the same should apply to diving. If you want to dive solo, go ahead! Everyone knows it is probably safer to dive with a buddy. But, really, not always.
That being said, since I'm a photographer, I guess I'm technically solo, as some have indicated. But my buddy is also a photographer and we are in the same water at the same time and usually within sight of each other. We know and accept this as a risk.
I like that we are taught buddy skills in class but as an adult, I like that I can cross a street alone if I want to, if you know what I mean.