Why would either of those be double-bad? They're the same thing as the original.
If you don't want to wear your seatbelt and you like to risk your own life, then that's on you. Just like riding a motorcycle.
They are presenting them as examples of things that a lot of people do say, and many laws are written this way, and many cops, prosecutors, & judges treat them as double bad.
And intentional killing is generally considered worse because it means you thought about it and then did it, vs when it's due to acute emotional disorder. Intentional crimes are usually treated more harshly
Owning a gun is legal, but if you've ever smoked weed it's illegal for you... murder is bad, but if it's because you hate a protected group it's a much harsher punishment... in some places, you can't be pulled over for not wearing a seatbelt, but you can be charged with it if you're pulled over for anything else. It's just to attach additional charges for prosecution.
I don't like the idea of excessive charges as part of the prosecution process in general. It creates/extends what I consider an unfair asymmetry between the state (prosecution) and the individual. That's not to say there aren't similar examples in the other direction, such as a clerical error resulting in dismissed charges altogether.