A reaction between Dexcool and any trapped air in the system or oil. Due to leaking gaskets.
Because it's cheaper to settle for this than pay legal fees when people keep coming at them. Look at the list of affected cars and all those engines are known for gasket issues. They took the smartest (cheapest) way out.
Because they don't know the real reasons behind why it fails. Or they're so set in their ways that anything new is the "devil" and will not work. Green stuff was dropped because it will adversely react with aluminum. Guess what, your engine has aluminum parts.
If you take care of possible leaks, keep your system well maintained (no air), Dexcool will be fine. But I will not use the old green stuff. I don't need it corroding any aluminum parts. If you have some sort of grudge against Dexcool, so be it. Use the universal Prestone stuff.