Because consent must be "freely given". As soon as you start attaching consequences unrelated to the utility itself, you're making a decision less and less freely.

The greater the power imbalance, the less free the choice. Social networks are a great example of this. You can choose not to use a particular one, but what's the alternative if everyone is already on that platform? You can go without, but what if it's LinkedIn, and there can be a real impact on your career?