They also prefix every PDF with a useless page telling you the authors (which are already listed on the first (now second) page anyways) and a list telling you which of the author's universities were members of ACM Open and paid for the publishing via flatrate.

The latter is of course the actual reason for this extra page, but it is also entirely useless information since the people reading the paper don't care. The people writing the paper are also usually annoyed by this (source: I'm an author of one such paper)