The DOJ is asking for either DirecTV or Turner Broadcasting be excluded from the deal, because being under the same roof would be a huge boost to DirectTV over its competitors - cable companies and Dish Network.
Turner Broadcasting includes several channels. Both TNT and TBS are probably more important to the deal than CNN (the former has higher ratings than CNN across the board; the latter does in primetime); the idea that it's the sole focus for DOJ is highly questionable.
Furthermore, how does DOJ's actual demand hurt CNN? AT&T could spin out Time Warner and keep the Turner channels. Or even if Time Warner spun out the Turner channels, they wouldn't go away (they were independent of Time Warner in the 1990s).
I'm as anti-Trump as the next guy, but the idea that there's not a rational basis for DOJ's demand is simply mistaken.