Think of it like this.
If you buy a certain brand of shoes (Nike), shirt, ect., you pay for it.
When they make another set of shoes, shirt, and change the color on it, you pay for that too.
“Dear Nike, I bought a pair of Blue Nikes, why I must pay for the black ones too?”
You see?
Only software company that I know of that gives free updates for Major versions is Image Line with their Daw FL Studio. I personally dont understand their logic/reason, but it works for them.