How we cite our quotes:
"You wanted to [come with the Tucks]?" echoed the constable, his eyes wide with disbelief. "You wanted to?"
"That's right," said Winnie unflinchingly. "They're my friends." (20.9-10)
Friends stick up for each other, and that's exactly what Winnie does for the Tucks. She totally could have thrown them under the bus, but she stands by them, protecting them from a pretty nasty punishment.
She had gone away with the Tucks because—well, she just wanted to. The Tucks had been very kind to her, had given her flapjacks, taken her fishing. The Tucks were good and gentle people. (21.3)
Winnie has a tough time expressing why she went with the Tucks. But we can all understand that we-just-wanted-to feeling. While it doesn't fly in some situations ("we just wanted to skip school"), when it comes to choosing friends, it's usually a solid bet.
"I don't know," said Winnie, "but it doesn't matter. Tell your father I want to help. I have to help. If it wasn't for me, there wouldn't have been any trouble in the first place. Tell him I have to." (22.23)
Friendship doesn't come with obligations. In this case, Winnie has to help the Tucks get out of a pretty sticky situation. Other considerations (like, say, her safety) go out the window.