Good question.

I suspect it's because human beings are very good at recognizing coercion when they're subjected to it and very poor at recognizing it when they're the ones exercising it. We often tend to judge our own actions by our intentions and other people's actions by their effects.

In other words, it's because principles are easy to defend when they benefit us and difficult to defend when they constrain us.

It's one thing to say, "I should be free to choose."

It's another to say, "Others should be equally free to choose when I dislike their choices."

Consistency is expensive.

Most people are willing to pay for freedom only when they're the ones receiving it.