his publisher told him the descriptions of same-sex behavior in a book on animal sexual behavior were against the law

So it wasn't legally or expressedly banned by the govt. But in authoritarian fear-based and legally unpredictable states it's not unusual for people to individually take rules even further than technically necessary, to affirm their adherence to the system and to be safe from even the smallest suspicion or most ridiculous accusation.

Might be worth noting that the publisher (Eksmo-AST) is one of the biggest book publishers in Russia.