No, we should not allow those links, but they are not necessarily piracy
Our scope should not include determining copyright claims on unofficial links to tell if something is piracy or not. You mentioned that these are "unmodified installers", but how are we supposed to determine at a glance if they are modified or unmodified? Even if they are unmodified at the time we check them, are we going to recheck them periodically and then take action when the site owner swaps the unmodified binary for a big bundle of malware or for a game client that has been subtly broken? What about when the unauthorized site gets a cease and desist letter from the devs and shuts down? This is the same problem as we have for answers that link without summarizing the relevant content.
Pokemon Go is challenging enough for us to troubleshoot as it is. Between strange UI choices, unstable backend servers, unreleased regions, geographical data differences, and actual game bugs, we have our hands full without adding "game sideloaded from questionable source" to the list.
Just as we don't support games from non-legitimate sources, nor should we encourage users to download games from non-legitimate sources. Instead, we should push them to get it from the official sites.