It's hard to say without knowing the setup of the league and other particulars, but I wouldn't call it absurd or league-ruining.
Basically the trade was a Stud for two solid starters. If a team were in a desperate situation and needed more depth at WR, the deal could make some sense. I wouldn't call it a good or fair trade, and I'd like to see someone get more for Johnson, but it would be reasonable
I believe that unless there is a clear case of collusion, all trades should happen freely in a league.