Except that all the people using left-pad weren't paying for left-pad, and didn't have a contractual relationship with the author. IANAL, but I'm doubtful the courts would find there is enough of a relationship for the author to be liable.
> nothing specifying that you need to continue making your open-source package continually and indefinitely available.
There's a difference between making it available, and deliberately causing harm and untold productivity loss in a single day. This was a case of the latter.