He doesn't do it for no reason I guess. But Walt has been treating Jesse like he's totally and completely incompetent throughout the entire series. And several times he's been wrong to do so. Yes it's life and death, and Walt being harshly critical of Jesse is one thing, but here lately he's gotten very nasty and condescending about it. Even disrespectful. After everything Walt and Jesse have been through together, and now Walt is going to start being mistrustful of him? As I see it, spying on Jesse was a pretty big betrayal. Yes, I'll reiterate that he has his reasons, life and death, serious business, etc., but he also has more than enough reasons to trust him.