Hosea 8:11

Because Ephraim has multiplied altars for sinning,

Altars have been to him for sinning.'

Meanwhile they have also continued to multiply altars at which they could sin (or ‘offer a sin offering'), establishing their altars ‘on every high hill and under every green tree'. Indeed all that their altars had done for them was to make them sin even more deeply. And this was true whether they were syncretistic altars at which both YHWH and Baal were worshipped, or altars merely for the Baalim.

We should not overlook the fact that according to Elijah there were a number of legitimate ‘altars of YHWH' in Israel which had been torn down because of the new Baal cult (1 Kings 18:30; 1 Kings 19:10), which may subsequently have been restored (without them there could have been no legitimate worship in Israel), but those are not in mind here.