This:
For, since the speculative reason is busied chiefly with the necessary things, which cannot be otherwise than they are, its proper conclusions, like the universal principles, contain the truth without fail. The practical reason, on the other hand, is busied with contingent matters, about which human actions are concerned: and consequently, although there is necessity in the general principles, the more we descend to matters of detail, the more frequently we encounter defects.
But Aquinas would agree with you that if there are cases where puppy-kicking is not cruel, then we cannot really say that puppy-kicking is cruel.
Yet if we have a rule that admits of exceptions then we can try to nail it down with a word like “fun” (or as I did here with the word “randomly”).