I do agree with you, but there is some indication for this idea in the suttas, namely MN60:
Moreover, since there actually is another world, their view that there is no other world is wrong view. Since there actually is another world, their thought that there is no other world is wrong thought. Since there actually is another world, their speech that there is no other world is wrong speech. Since there actually is another world, in saying that there is no other world they contradict those perfected ones who know the other world. Since there actually is another world, in convincing another that there is no other world they are convincing them to accept an untrue teaching. And on account of that they glorify themselves and put others down. So they give up their former ethical conduct and are established in unethical conduct. And that is how these many bad, unskillful qualities come to be with wrong view as condition—wrong view, wrong thought, wrong speech, contradicting the noble ones, convincing others to accept untrue teachings, and glorifying oneself and putting others down.
To me this seems somewhat of a slander, a doctrinal position posed by Buddhists to paint materialism in a bad light. We see similar debates between Christians and atheists nowadays, where the former wonder how the latter can be moral without a belief in God, even though most of them clearly are. And overall probably no less than Christians.
Elsewhere (e.g. DN2, DN23) the materialists are said to teach that “there are no results of good and bad deeds”, but in my view this specifically refers to post-mortem results. They can still have a moral system regarding results in this life. Some authors have argued for this as well, based on ancient Hindu texts that talk about materialism mentioning certain rules and virtues; sorry forgot the reference and don’t have it here.