It was apparently because he was a no show to a scheduled fan meet and greet, and because he didn't play a single minute in the friendly when Juve were contractually obligated to play him, which understandably drew the ire of fans. The Korean fans know they got screwed over, especially since they sold out a 60,000+ seat stadium within a couple of hours for a friendly, even though the tickets ranged from ~$100-$400 per seat (or so I've read), and obviously a big reason for that was to see Cristiano.
I think the tour organizers are partly to blame for this, and I can understand Cristiano deciding not to play even if he had a contractual obligation (since you don't want to over exert yourself in preseason and potentially get injured), but it probably wouldn't have hurt if he displayed a little fan service. It's an overall bad look, and he probably lost a lot of fans there, not that'd he care I suppose. Just typical of modern day football though, it's all about the money and milking fans for all their worth.