In the first book (James Potter and the Hall of Elders Crossing) of the James Potter series (a fanfiction that J. K. Rowling liked) by G. Norman Lippert, Slytherin house gives James Potter and standing ovation, before he went to the Griffindor house. They were also very kind to him since the beginning.
I don't get why they are kind to him while they are members of the P.E. group (Progressive Element).
From James Potter and the Hall of Elders' Crossing:
It shocked and worried him for a moment. He was irrationally afraid that somehow his thoughts had gotten out and betrayed him, that it was the noise of the Gryffindor table standing, booing him. But it wasn’t the sound of booing. It was the sound of applause, polite and sustained, in response to the calling of his name. James turned to the Gryffindor table, a smile of gratitude and happiness already lighting his face. But they weren’t the ones applauding. They sat there rather blankly. Most of their heads were turned toward the source of the sound. James turned, following their eyes. It was the Slytherin table.
Tabitha, the girl who initiated the applause, later approaches James and seems to want to be friendly and put old enmities aside:
“No, no!” the pretty Slytherin girl said, raising her hand toward him, palm out, stopping him in his tracks almost as if she’d used magic. “Don’t stand. I’m happy to see you feel comfortable enough to sit at the Slytherin table with us. These are quite different times than those of your father. But I assume too much. Mr. Deedle, would you be so kind as to introduce me to your friend?”
And indeed, Tabitha was behind the Progressive Element:
James hadn’t, actually. “Doesn’t really matter. It was the Slytherins that got the whole thing started, with the Progressive Element slogans and badges and stuff. You said so yourself, Ralph. Tabitha Corsica was handing the badges out to all the Slytherins. She’s behind the whole thing.”
It seems likely that the purpose of the applause, which was initiated by Tabitha, as well as Tabitha's own overtures toward James, were meant to make him sympathetic to her cause.
If the son of Harry Potter, the hero of the war against Voldemort, could be seen to endorse PE ideals, it would be quite the coup for their group. Even if he was merely seen associating with them, it could be a potent propaganda tool, for recruitment and other purposes.
