I think it was the lesser of a variety of evils, but only just, and it's still a dreadful decision.
They could have brought back Johnny, but I've decided I'm glad he left on a high with everyone rooting for him and wanting more from him, and didn't risk leaving on a less happy note or being rejected all over again.
They could have chosen someone to bring back without a public vote, like a contestant that recently left the competition. But, seeing a face we thought had been publicly voted out wouldn't generate anything but bored irritation. It wouldn't help the act to be voted out straight away, twice, and it wouldn't help the show to reverse a decision reached with the help of public votes. It wouldn't feel like a "twist", it would just be weird.
They could have brought back no one, and had a vote free show, but that would have made for a very boring show and isn't a realistic choice given that they would have lost the money from voting.
This way, they stir up lots of debate and get people talking about the show, potentially get to spice up a rather boring line-up of acts with someone new, generate more money from votes and distract people from the absence of Frankie and the disaster that entire situation was for the show.
Unfortunately, it means completely shafting the people who have competed so far for their place in the competition! It completely stinks and essentially I think the show managed to back themselves into a corner where there were no good option left.