It's due to the parasocial relationships, they feel like it's cheating and there is of course Idols who really hate knowing you go to other Idols as well. Some will only act jealous as part of the chara they play (and there is fans who want a jealous Idol), but some really are. It's a tough business after all and every fan counts.
Nightmare of the average AKB-otaku in my twitter timeline: Both his Oshis having their handshake lines right next to each other, so they can see them going to both. lol
I didn't even notice we didn't have volume 4. That explains why I remember cropped panels and screenshots from the manga posted on /a/ that we never got translated.