Have you ever noticed a common trend in the Fighting type??? They are all physical attackers, which means at least one of my non-fighting Pokemon has to deal with being the special sweeper, which leaves me with one Pokemon to deal with those 6. Trsut me, I tested out every fighting Pokemon in UU over the last few weeks since Tori told me about this thing, and I tested tonnes of teams, and the one common thing in all the team is that if two or more of these Pokemon came together they never stood a chance. On top of this, I always felt 5 of the 6 were stretching UU anyway, and then with Zam getting in there, it felt if I am going to be restricted to a really small group of Pokemon then I need to level the playing field. I also consulted some top players about this 6, one of which is on the smogon team, and he felt it was fair. Dug and Espeon may not last in it however and it may become 4. I have some more testing to do with it later.
The other reason for this rule is because these 6 are way to common. The big 5 rule for OU back in the day was mainly made for the killer combination the big 5 made, but as a side note it was noted that their overuse was a motivating factor, and I did join this league to face new teams and not the 5 or 6 most powerful Pokemon there is in the tier. Trust me, as Sky said last night to me, if there was a way to counter them all in the fighting gym without compromising the other holes to cover up such as weaknesses and lack of walls and special attackers, I would have figured it out by now. Anyway, this is all trial, if the rule turns out to make thing too easy for me, I'll be the first person to say it has to go. But things don't work until they are tried.