They don't have to. But if a sponsor is doing something that someone fundamentally disagrees with, they shouldn't be told to shut their mouths so the money keeps rolling in. That's called corruption.
Compromising your beliefs for money is a form of corruption. It's just one that people are forced to do every day to varying degrees.
From Wikipedia[1]: 'In philosophical, theological, or moral discussions, corruption is spiritual or moral impurity or deviation from an ideal. Corruption may include many activities including bribery and embezzlement.'
Keeping your mouth shut in exchange for sponsorship money sounds a bit like bribery to me.
