The position that criminals should be punished in proportion to the severity of their crimes but that repeat offenders should receive harsher sentences is flawed. It wrongly assumes that past misdeeds are relevant to the seriousness of an offense. This introduces a vague standard that would make almost every other consideration relevant, making it impossible to apply this proportionality principle.

The statement in the question stem is a consequence of the “unsustainable” position that the author’s refuting. The author’s saying that if you take that unsustainable position, it would lead to a series of questionable implications.

Note that the statement in the question stem, taken on its own, is not support for the argument. The author implicitly disagrees with that statement. Rather, it’s the fact that the unsustainable position implies the statement in the question stem that serves as support for the argument.