That'll depend on the field. In computer science, where your example comes from, there is some competition between publishers for conference proceedings and also, to an extent, competition between conferences for quality submissions and high-profile researchers on the committee.
Some conferences are moving or considering to move to open access proceedings, and this makes commercial publishers try and offer better deals, which includes better attitude towards authors. PLDI in your example is ACM's own conference and won't just go to another publisher, but still, harassing authors with copyright restrictions is going to repel authors and committee members. I know, universities and grants in Europe often require to upload publications to open access archives, but I don't know whether this would prevent authors from publishing in a venue that does not allow to publish preprints, since this problem does not normally arise.
I imagine that this indeed hurts publishers. For example, at some point in the past I did have access to Springer's LNCS in my institution, and these days there's some story unfolding in France with universities not renewing their Springer subscriptions.
I'm also curious to know where this will lead. Perhaps, publishers will soon start tightening their copyright agreements (thus confronting the existing open access policies). Or perhaps, they will raise publication costs. Participation in a CS conference is already expensive, usually somewhere between 500-1000 Euro just for being able to present (and then there are travel costs), so it may not make a big difference if a larger portion of this money goes to a publisher.