Well, the magic number is that "last 30 days" rating. Think of it like this...If I'm an 1840, then quit playing for awhile and timeout all my games, I could drop to 1145. Now, if I didn't play for 30 days then I could enter the rookie duel (as my "last 30 days" rating would be 1145). As soon as I start playing in the tourney, I smoke everyone and my rating skyrockets past 1250. This is most likely how your opponent got into the rookie duel.
Unfortunately, there are some unethical players on this site and you will see many threads on various tactics that they employ. What can you do about it- nothing - or maybe have Adam throw up the apple.🙁 There always has been unethical people in this world, always will be.
I am in exactley the same position you find yourself in, only in the 1500-1599 3x3. The result will be the same, I will lose both games, and with it the opportunity that i might, just might have had a chance of winning a tournament.
Yes it is annoying, but there is not much I or you can do about it. For the record, the opponent in question is a desent person, I do not think that in this case they had purposly dropped their rating just to enter this competition (I agree some do for all the reasons you highlighted). They just left the site for a period of time.
Ratings are an odd thing, when good players have a low or developing ranking better players will not play them as they are scared of lossing rating points themselves, while they themselves seem more than happy to beat up lower ranked players for "easy" points. This would be fine but for the real odd complexion, when these players then get up to a 1600 plus rating they no longer seem too happy to continue to play the weaker players on a regular basis.