I don't see the problem in an idol expressing that she likes otheri idol groups, it doesn't mean that she dislikes the idol group she's in, it just shows she has interests in other groups as a listener. It is a shame H!P kind of isolate the girls to not express their love for other idol groups.
I think the specific issue with MM lies with the fans feeling "threatened" by AKB. Probably a lot of the fans have been around since the early days when Momusu was the top idol group in Japan, and those fans harbor animosity towards AKB for "stealing" Momusu's position (not saying I agree with it), so any mention of AKB in a positive light would be, to those fans, like a stab in the back. Newer fans who might not think that way likely conform to that thinking and it becomes an unfortunate cycle. Since idols are built on a foundation of catering to fans, if something makes fans mad, then management is going to prohibit it. Is it stupid? Yes. Is it immature? Yes. But that's the way it is. It's the same reason idols have the no dating rule. But again, from that short preview we really can't know what exactly was said in response to that question.
Btw, that gif is hilariously awesome. Did you make it?
Edit: ^ It would be a foreign fan.
I understand what you mean and I agree if Morning Musume expresses love for another idol group it makes the fans think they like other groups better than their own and it has a bad outlook on the idol group in general. But it would be nice to see which idol groups Morning Musume like. The fans ofc think Morning Musume is the #1 idol group and ofc they will be threatened when groups like AKB are thundering success with their group. So if a member announced they loved AKB48 it would make the fans think that they are fraternizing with the enemy It's a shame that fans can be very hostile when it comes to the case of H!P fans disliking AKB48 I mean who cares if AKB48 is more popular? I can understand that they want MoMusu to be the top idol group and I wish for that to happen to but I am just happy that they are still releasing music and are still successful. I know the idol industry does isolate the girls a lot with dating, but I wish it didn't get so isolated that they can't even express their musical interests in case it causes a scene in the public eye or in the fandom. As much as I love idols sometimes I think the business side of it is a little too much But it will never change no matter what I think of it hehe! And I wonder if idol fandoms were a little less, how do I put it hostile?, then maybe the business side of it wouldn't be so isolated. I mean what if fans didn't care if their idols dated? Would that still mean the dating rule would exist?
No I didn't make the gif myself, I found it on tumblr and found it amusing