An integer nargs value is only used in one of 2 ways,

range(nargs)

'%s'*nargs

In both a negative value acts the same as a 0.

I don't think the original authors though much about 'what if the code user gives a negative value?', because nargs is counting things - the number of expected arguments.  For some actions that number is 0.  For other some sort of positive integer, or variable numbers like '*','+' make most sense.

To some degree nargs is modeled on the regex sequences, '*','+','?','{n}'.  '{-1}' does not produce a regex error, though I can't make anything match it.