Not necessarily cheating, but consistently playing against a very limited group of opponents provides no indication of strength against random opponents. When/if such a "private Elo" reaches abnormally high values (i.e. a person is in top 20), it might cause tension among actually strong players, and at that point it is typically reset by admins. While not ideal, this seems to be better than many alternatives. Probably there is a way to automatically cap "private Elo" at, say, 299, but I don't see how to easily implement it and communicate the reason for the cap to folks affected by it.
The main point is - "private Elo" isn't comparable with "public Elo", thus when it is very high, it isn't actually earned, and when it is reset, there is nothing to be upset about.
The main point is - "private Elo" isn't comparable with "public Elo", thus when it is very high, it isn't actually earned, and when it is reset, there is nothing to be upset about.