‘Rejoice not, O Israel,
Do not shout for joy, my people,
For you have played the harlot, from your God,
You have loved hire (hired women) on every grain-floor.'
Hosea warns Israel not to be so full of joy at their harvest feast at the Feast of Tabernacles because they needed to recognise that by their adulterous behaviour they have made God angry. Such ideas of joy were seen as being typical of the Jewish feasts, which were in the main joyous occasions (Leviticus 23:40; Deuteronomy 12:7; Deuteronomy 12:12; Deuteronomy 16:11; Deuteronomy 16:14). But they were not propitious when their behaviour was lacking.
‘Do not shout for joy, my people' is obtained by repointing the consonants in the original Hebrew text. (The vowels signs, and division of the words, resulted from the work of the Masoretes some centuries after the time of Christ, and are not part of the original text. Following the MT we would translate, ‘Rejoice not O Israel, to exaltation like the peoples' with the idea being that they should not exult like the nations as they would be treated on a special basis because they were God's faithless people). The reason for their rejoicing was because they considered that their abundant harvest demonstrated the satisfactory nature of their religion. After all, they no doubt said, it had worked, hadn't it? But Hosea was pointing out that their joy would be short lived, because in the near future there would be no such harvests.
The charge against them was that they had ‘played the harlot from their God', either by worshipping Baal and Asherah, or by worshipping YHWH on a false basis by using cult prostitutes and trying to ‘move Him to action' by their adulterous behaviour. Either way they were being faithless to YHWH's covenant, and therefore behaving like an unfaithful wife to Him. They had been warned about such behaviour in Exodus 34:15-16; and it was the kind of behaviour prophesied of them, along with the consequences, in Deuteronomy 31:16.
‘You have loved hire (hired women) on every grain-floor.' God's charge against them was that they ‘loved' their cohabitation with hired cult prostitutes which they were involved in during their ritual activities wherever they occurred, which was regularly. Grain floors were wide open spaces suitable for gatherings (and for sexual activity), and it is very probable that they were widely used for religious activity, a certain sacredness being see in them as the place where the grain (the gift of God or of the gods) was finally made edible and provided in abundance (see 1 Kings 22:10; 2 Samuel 24:18).