Musar על ברכות 31:13

They likewise exempted hired workers from reciting the (Hamotzi) blessing over bread and from the last blessing of the grace after meals (Birkat Hamazon). And even in the case of reciting the Shema, they required them to pause from their work only for the first chapter (Berachoth 16b). How much more so for things that are optional. And if he transgresses this, he is considered a thief.