It's because he's an Israeli. He didn't join the organization before joining the army. That happened after the fact. It doesn't matter who funds the organization. They're not forcing him to say anything he doesn't want to say. That's ridiculous.
U.S. says working on initiative to de-escalate Syria fighting, with focus on Aleppo (Reuters)
from the article: IDF soldiers: Problem in West Bank isn't Palestinains, it's Jews