There are many reasons for a politician to say nice things about Islam. First of all, imagine the opposite. Imagine what would happen if a prominent politician, like the president of the United States, were to say, "ISIS is following Islamic doctrine to the letter." What would happen? In the politically-correct climate we live in right now, he would be destroyed by the mainstream media. Op-Eds would criticize his insensitivity and would accuse him of racism and Islamophobia. Organized Muslim groups would also make a big deal out of it, losing any Muslim votes for that politician forever, and probably losing a lot of sympathetic non-Muslim multiculturalist voters as well. Prominent Muslims with doctorates in Islam would authoritatively and publicly convince most of the non-Muslim population that the politician was completely mistaken.
It's also entirely possible that the politician's honesty would set off riots around the world. People might die in these riots, and the politician would be blamed for it. When cartoons of Muhammad were published in a Danish newspaper, 187 people were killed in the ensuing riots, and the editor of the paper has been hunted by devout Muslims ever since. He has already narrowly escaped one assassination attempt.
Contrast all those consequences with simply saying the acceptable thing: "Islam is made up of peaceful, law-abiding citizens and these ISIS barbarians don't understand true Islam." There are almost no negative consequences from this. A few scattered counterjihadists grumble a bit. And that's about it.
I believe most politicians genuinely believe the politically-correct nonsense. And most of the few who have their suspicions about Islam will keep their mouths shut until the political climate changes on that issue.
What will make the political climate change? More citizens who understand what Islamic doctrine really says. When enough of us know the facts, a politician who speaks honestly about Islam will get enough public support to stay in the game.
So if you would like to see politicians tell the truth, the best thing you can do is educate your fellow non-Muslims about Islam.