From the perspective of a singer/voice major:
It would be STUPID for idols to sing live 100% of the time in the settings they sing in and in the style. I don't think people understand that pop music and it's counterparts are incredibly tough on the voice. There is obviously technique to ever style of singing, but sometimes pop music requires you to disregard some of those techniques which can be a strain to the voice and damaging overtime. I do opera and classical. There is clear cut technique for what I do. My vowels follow IPA, my breathing technique is written in the books. But pop music is incredibly diverse and requires you to "stand out" and "break the rules" to be noticed more. Thats why we have all of these voice cracks and artists that sound like they're dying when they hit 35. Especially Kpop idols who are expected to belt early on in their debuts (Such as EXO with Mama, though their voices have progressed beautifully and they've been doing more music individually suited to their talents and vocals which is good to see). You can tell who focuses on technique in Exo. I would absolutely love to be a voice coach for a label over in SK just to learn what they're learning.
It's healthy to give your voice a break. Nothing wrong with lip syncing. From a performance aspect, that gives you a chance to focus on mainly the visual aspect of your performance and find weak points in that aspect which can contribute to a better performance overall. The only time I see an issue with lip syncing is if it's done out of laziness. But that's not usually the case. Voice breaks are healthy!!