And by charging the authors to publish the paper in the first place. Although I think a lot of the money both ways comes from universities and other organisations that pay for publication costs and have deals with publishers.

Thank god for CS's culture of pre-prints on arXiv.

Yes, that's got to be part of the explanation. What's not clear to me is why anyone gets professional credit (e.g., tenure) for publishing in a journal that is obviously bad.

The answer to your question is just part of the whole sad story...

Promotion committees (hopefully!) know which journals or publications are low quality. If they are unfamiliar, they generally don't have time (or inclination) to verify the paper(s) or publication itself. Especially true when it's a slightly different sub-field than their own. So it often comes down to a simple count of pubs. That's usually all the credit they need!