If we’re talking about things being made clear, look at it from Marietta’s point of view then. The girl made it very clear she didn’t even trust the situation, nor want to be there. If someone is making themselves that obvious, don’t let them join, it’s just asking for it. Then, if Cho for example, got sneak written across her face for telling Marietta club details and whatnot, then I’d understand, because Cho knew and believed in the risks, yet would still choose to tell an outsider who cannot be trusted with this information, but that wasn’t the case.
Marietta was specifically told by her parents not to get involved with anything that would bring about Umbridge’s ire. Cho too, but unlike Marietta, Cho had something to fight for and believe in. Marietta joined, yes, but we know it was more for Cho’s sake than not. Of course WE knew Harry wasn’t a liar, but not everybody knew him like that or trusted his words, whether out of fear of facing the truth or whatever the case may be, Marietta was one of them and her parents likely were as well, as they worked for the ministry.
Marietta told what was happening, likely to keep her nose clean in order to protect her family from getting into trouble, for something she didn’t believe to begin with, and perhaps to stop Cho as well, because while Cho was there as a means of protecting herself and her loved ones, her greatest goal was to avenge the death of Cedric, whom Harry said was killed by Voldemort.
As Cho’s friend, I’m thinking she disliked what she believed to be Harry’s lies, because it encouraged Cho to join an illegal organization, her friend has already been shown to be vulnerable, grieving over the death of a loss, and from Marietta’s POV, it’s likely she believed that Cho was choosing to latch onto the idea that someone was behind Cedric’s death, despite the fact that it was announced that his death was a tragic accident. It’s easy for people to place blame and it’s even easier to place it on a mass murderer. Marietta likely felt Cho was falling into that mindset as a means to try and gain closure for a loss, by killing whoever killed Cedric.
She was fifteen years old, and she now has a permanent scarred face with sneak written across it. Except she wasn’t being a malicious sneak out to destroy something for the sake of doing so, she was still a child, trying to decide between loyalty to her grieving friend and to her family.
If they wanted to protect the secret, a tongue locking jinx would have been more efficent, this was just underhanded. It wasn’t protecting the secret either, because Marietta was able to tell, it was a way to publicly humiliate anyone who told on the group, a permanent scar they have to carry for life. A way to let everyone know this person cannot be trusted.
War was not in her mind, so to her, it didn’t matter if she told on some club that was illegal by the government to begin with. That’s why I can understand Marietta’s choice. Obviously it wasn’t right because Voldemort was back, but if she truly believed that, I doubt she would have told.