The ACM grants back " The right to post author-prepared versions of the Work covered by the ACM copyright in a
personal collection on their own home page, on a publicly accessible server of their employer and in a repository legally mandated by the agency funding the research on which the Work is based. Such posting is limited to noncommercial access and personal use by others, and must include the
following notice both embedded within the full text file and in the accompanying citation display as well: ..."
It doesn't seem fair. They charge a fee for others to read your article, but your rights to the same (your) article are limited to free access and only under limited circumstances.