“Do not rejoice, Israel, with jubilation like the nations!” Israel was worshiping pagan gods, and attributed to them the blessings that actually came from Yahweh (Hos. 2:5, 8), and so they rejoiced “like the nations” at the grain harvest, honoring pagan gods.
“You love the wages of a prostitute at every threshing floor.” A prostitute could be paid with grain, and God is saying that Israel committed prostitution against Him and served pagan gods expecting to be paid by a bountiful harvest. Also, however, it was common for prostitutes to show up at threshing floors. They knew the men would generally be away from their families, and they could pay right away with grain. If the guard was a servant and not the landowner, they might be able to be enticed to hire a prostitute for some grain they would sneak from the pile, and no one would have known it.