Well,one could argue that the penalty was inappropriate and/or excessive but she *was* speeding.IIRC she was doing something like 75 in a 65.
While it is often synonymous with a fine, it differs in that a fine is a fixed sum prescribed by statute and was often voluntary, while an amercement is arbitrary.
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The reason I maintain that it's "arbitrary" is because usually the peasant is not the only one "violating the law" in instances like yours.