Why is a beard shaver judged to be sinful, when some reputable scholars are of the view that growing a beard is a Sunnah and not an obligation, and why is one who follows the opinion that it is a Sunnah condemned?

The saying: "There is no denunciation for abandoning a mustahabb (recommended act) or a matter of ikhtilaf (disagreement)" is not absolute. Rather, it is restricted by the condition that it does not contradict a text from revelation or a consensus (ijma'). As for the saying: "There is no denunciation in matters of ijtihad (definition) (independent reasoning)," this is correct because ijtihad only applies to matters for which there is no explicit text. What is transmitted from Imam Al-Shafi'i regarding shaving the beard is prohibition (tahrim). Although An-Nawawi and Ar-Rafi'i inclined towards karaha (dislikedness), Ibn Ar-Rif'ah objected, stating that Al-Shafi'i explicitly mentioned tahrim in Al-Umm. Even if the disagreement is considered valid, the absence of denunciation for one who shaves it does not mean that one should not clarify that the preponderant opinion is prohibition.