Radak: The custom is that passersby say so to harvesters, like Bo'az said to the harvesters "Hashem Imachem" (Rus 2:4). They will not say so, for roof grass is not seed of Brachah, and it is not harvested. So will be the Nochrim - all their Torah and belief is folly and empty, like roof grass, and so it will be finished off like roof grass. However, Torah of Yisrael is compared to seed of Brachah; refer to 126:5:1:2, 126:5:2:2.

Malbim: The custom is that passersby bless harvesters. When the grain is blessed, they say "Birkas Hashem Aleichem." At a time of famine, they say "Berachnu Eschem b'Shem Hashem" - the Brachah should take effect from now and onwards. Passersby will not bless them, for they will be cursed, and not blessed.

Radak: The matter is repeated.