The reasons teams that play only one event are not counted towards relegation are simple...
1) The first day of competition at HG3 was a league point scoring day. Now all teams are entered on to the tables in case they decide to play more events later in the series. If they weren't, their points at HG3 could be lost and that just wouldn't be fair. However, HG3 attracts teams that do not play the league so to relegate them would be meaningless and pointless.
2) If we did include teams that have only played one event in the relegation assessment then teams competing in the series for more than one event would be very unlikely to get relegated and that would make a mockery of the promotion and relegation system.
It does work. You see, for teams that are looking to compete - as Impact UK were at the start of the season - non-attendance does mean you loose out and fall closer and closer towards relegation for each event you do not attend. This is how it should be - if a Premier League football team didn't play its last five matches of the season because they could not field a team, you would see similar results.
Similarly, those teams that do commit to whole season are rewarded as they should be.
I don't think your argument makes any sense, though I am sorry you feel cheated somehow.
The system isn't perfect because the series does not yet have the luxury of having enough teams to enjoy a locked division format. Hopefully, next year's merger will go a long way to seeing that a reality. However, it is (in my mind) the best system to use given the circumstances.