Just happened to see this segment in a piece from the Mevakshei Torah journal in the name of Reb Dovid Povarsky Z”L.
He brings from a Rashba the following question. How come when Chazal said not to blow the shofar on RH when it falls on Shabbos, it does not violate Bal Tigra (not to subtract from mitzvos)? The Rashba answers that takanos do not violate Bal Tigra.
But Reb Dovid asks on the Rashba, look, we know that there is a mitzva to blow shofar, yet Chazal are allowed to be oker (uproot) a mitzvah b’shev v’al taaseh (passively, not to do the act) to protect Shabbos. So why is the Rashba any more bothered by Bal Tigra which is also passive, then he is about the mitzvah of shofar in the first place. If shev v’al taaseh is a good enough heter for not blowing shofar, it should be good enough for Bal Tigra. Reb Dovid answers that apparently, a passive act by Bal Tigra is not considered Shev v’al Taaseh, but rather Kum V’aseh (active violation).
It seems to me that the pshat in Reb Dovid’s words is that by definition all Bal Tigra is passive, i.e., omitting a mitzvah. So if the Torah asurs it, it must mean that even passive acts are considered an active violation of Bal Tigra. The Rashba must therefore be mechadesh that by takanos, Bal Tigra doesn’t apply.
(Not a parsha vort, but thought was a geshmaker kashya.)