Just to play Devil's advocate (lol), what about the times Kasumi confiscated or deleted Sakurako's collections of secret pics and tapes? You know, all that Kasumi-themed memorabilia Sakurako had acquired by secretly photographing or recording Kasumi?
Well, those were played for jokes and solved in one strip, two at most. It still wouldn't fit the "central unit of the plot" definition.
Also: Sakurako's attitude. She acted like she knew she had been naughty, and done something she shouldn't have. And it's easy to see why.
I recall a comic I read where a girl was bragging about her boyfriend. He was really nice, and attentive, and accomodating; he took good care of her and pampered her, even when she asked for selfish stuff. For example: she had a cosplay hobby, she liked to dress people in costumes, and he was ok with it and willing to let her do all she wanted; so she often dressed him up as a fairy, with a skirt and big butterfly wings, and she took pics and put them on her website. These pics were very popular; she posted the links in forums for the drag fandom, and her site got lots of hits. The other girl, after listening to all of this, was very impressed:
"Wow! He even lets you take pics of him in fairy drag? And upload them to your website? He must really love you!"
And the first girl, making a coy face, replied:
"Weeell... he doesn't know yet about the website..."
This is a good example of why, in times of the internet, you don't want those pictures or recordings of you that you don't want to go around to be out of your hands. That's why it's reasonable for Kasumi to be not happy to find that Sakurako took pics of her and filmed and recorded her without her knowing, like when asleep or in the shower; and also why Sakurako, caught being naughty, can't really complain when Kasumi takes that stuff from her.