Common practice often teaches us about halakhah. When authorities debate an issue and common practice follows one opinion, this teaches us that the Jewish people ruled like that view. This decision-making method has halakhic force in deciding between existing positions.
A kohen is obligated to recite the priestly blessings, birkas kohanim, on his fellow Jews every day. While Ashkenazim outside of Israel generally only recite it on holidays, a non-kohen may never perform that mitzvah. Yet there are many non-kohanim who recite the blessings as an enthusiastic farewell or bestowal of good wishes. Are they behaving improperly?
The Mishnah Berurah (178:3 and Bi’ur Halakhah sv. de-zar) connects this practice to two debates. The Bach rules that a non-kohen only violates this prohibition if he recites the blessings while raising his hands like the kohanim. However, the Pri Megadim disagrees and forbids any recitation even with lowered hands. While one might have thought that this prohibition only applies to reciting the blessings as part of a prayer service, the Mishnah Berurah rejects such a notion. The prayer service, and according to some even the obligation to pray, is only rabbinic while the mitzvah of the priestly blessings and its attendant prohibition on non-kohanim are biblical.
A long-running debate that the Talmud fails to conclusively resolve is whether you must have proper intent to fulfill a mitzvah. If you do it without realizing you are performing a mitzvah, do you have to repeat it? If you hold mitzvos tzerikhos kavanah then you must and if not, then not. Similarly, if you need to have intent in order to fulfill the mitzvah, then your lacking that intent successfully avoids the prohibition of a non-kohen reciting the priestly blessings. Because you did not have the proper intent to fulfill the mitzvah (if you could), then you also do not violate the prohibition.
Perhaps, the Mishnah Berurah suggests, the common practice proves that we follow the view that you need to have proper intent to fulfill a mitzvah. While this position normally leads to stringencies, in this case it creates a leniency because it allows people to recite the priestly blessings because they lack the intent to fulfill a mitzvah. Or, perhaps, common practice demonstrates that we follow the Bach‘s view that the prohibition only applies to a recitation while raising your ands like the kohanim.
The observant community should be assumed to follow halakhah and the practices of the pious maintain a presumption of propriety. When a questionable practice can be explained by a pre-existing view, we must assume this as a basis for normative behavior. In this specific case, though, other explanations can resolve the questions.
The Mishnah Berurah offers a more complex possibility. Since the Sages formally connected the priestly blessings to the prayer service, someone who recites the blessings outside of that framework is considered as if he specifically intends not to fulfill the mitzvah. Therefore, even if we hold that the prohibition applies even when you do not raise your hands and that you do not need to have proper intent to fulfill the mitzvah, in this situation it is as if you specifically intended to not fulfill the mitzvah and therefore you avoid the prohibition.
R. Avraham Erlanger (Birkas Avraham, Kesubos 24b sv. nesi’as) takes this last approach in a slightly different direction. Since a non-kohen reciting the blessings in a way that would otherwise fulfill the mitzvah is forbidden, we can assume that he has intent not to fulfill the mitzvah. Unless a non-kohen explicitly recites the blessings for the sake of the mitzvah, halakhah assumes he intends not to.