Hidetsugo says that "instant or sorcery 'may' be played"- yet should not enable someone to cast a sorcery on opponent's turn!!!
Sorceries may only be cast on one's own turn- so Hidetsugo should not enable someone to violate game rules.. someone using a Hidetsugo cast to proc Breach on defend, seems to me an exploit. What next?- a card that reads "upon casting you 'may' win the game" and then the person playing it immediately proclaims victory??? The text 'may' should never imply capacity to violate game rules- especially when the card doesn't explicitly state that "You may cast an instant or sorcery card revealed in this way, without paying it's mana cost. A sorcery cast in this way may be played on opponent's turn."
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[Deleted User] commented
The game rules do not prohibit a player from casting a sorcery with non-sorcery timing. The game does give permission to cast sorceries with sorcery timing. Card effects can give other permissions to cast sorceries. In order to cast any spell or activate any ability, you only have to have permission to do so. Hidetsugu and Kairi's trigger gives that permission.