From a comment on reddit which breaks this down a bit further-

Devil's Advocate: In Japan, being an "idol" is profession. It's a profession like being a pop star, a singer, an actor or a radio show host is. These girls know exactly, exactly what they're getting into when they sign their contracts with their talent agencies. We in the west like to compare it to a young pop star. While there is some overlap, they are still different jobs with different requirements. Idols know they can't date or they'll ruin their image, they know they can't be seen smoking cigarettes are getting drunk publicly. Most of them have no problem with that because they're under the age of 20 and they can't legally do those things anyways. Some girls as as young as 11 years old, though they're usually "in training" until 15-16. I don't think an 11 or 12 year old girl should be drinking, smoking or having sex no matter the circumstances.

I'm not religious but imagine if a catholic priest was caught fornicating with women, getting hammered at bars, or gambling at a casino. How would that hurt his reputation? Yes, he should be able to do whatever the hell he wants to do within the bounds of the law, but he's just ruined his reputation and put his job on the line. He knew what he was getting into when he signed up to be a priest and he knew what was expected of him. It's not something that was forced upon him. Idols are exactly the same. They're literally being paid to be "pure" for their fans. They signed contracts with their talent agencies specifically forbidding those kinds of behaviors. They didn't have to sign it, everyone in Japan knows what's expected of an Idol, even children. If I sign a sponsorship with coke and am told I can't be seen in public drinking pepsi products, that's not some taboo thing, it's just business. It's good for me (I'm being paid good money), and it's good for the corporation (I'm helping sell their products and making them money too).

When an Idol tarnishes her image in that kind of way, she's like the priest seen out partying. She likely damaged her career and cost the talent agency a significant chunk of money that they had invested into her thus far.

The kicker is that not all young japanese female celebrities are idols. There is overlap but they are different from actresses and pop stars. Japanese pop stars and young actresses can and do have boyfriends and are seen out partying all the time with no outrage. It's because when you sign up to be an idol, you usually don't have the talent for much else. You just have your looks. So potential idols sign a contract stating they'll not do those things because they want to be famous. They can't sing, can't act, aren't funny, so for a shot at being famous, their "talent" is going to be to devote herself to you, the fan. An idol dating is like an actor showing up to work and then telling the director to fuck off, they're not acting anymore.

I think the confusion comes from the fact that we don't really have anything like an idol that compares. People try to compare them to pop stars but that's simply not accurate. Even in this very thread, there is a comparison to Taylor Swift when an idol's actual job is actually probably closer to a priest or some other job that requires you to be a pillar of morality. That's literally what the job requires of you. Kind of like how being a singer requires you to sing, or an being an actor requires you to act. Being an idol means you're there to be idolized.