Comment by jules

That isn't the case. Conferences organize their own website to submit articles for review. Volunteers from the conference pre-filter submitted articles for spam, the rest is handled by the review committee. There is no cost to submit. In fact, the eventual cost is often not even mentioned at that point. When the article is accepted for publication, the conference gives authors a link to an ACM website where the authors upload their PDFs. Only after that will the authors be asked to pay the fee (and if you wanted, you could refuse at that point, which presumably means that the conference will eat the loss, or maybe they'll un-publish your article).

I don't think spam is a huge issue. The conference websites and submission portals are niche and random people don't tend to find them or care enough to go through the trouble.