I think the problem is that this is too easy for controlling or abusive people to twist around to their benefit.
For example, they could say, by walking out and not talking to me (which feels like punishment and not being heard) when I'm shouting, you are trying to coerce me to change my behaviour, ie not shout, hence controlling.
If you threaten to leave me if I don't stop drinking myself into oblivion every night, you are trying to control my behaviour, because you know I don't want you to leave.
Other side of the coin, they could demand something completely unreasonable, eg you can't ever see friends without me being present, and claim it's them setting the boundary that they don't wish to be with someone who wants alone time with others. Even something completely normal, but they try to convince you it's not normal to do that in a relationship and threaten to leave and upset the family dynamics, or walk out and refuse to talk to you. And most people want to avoid that so abide. But it's not controlling, because he is allowed to leave someone or not engage in conversation when that person behaves in a way they doesn't like (same as in the earlier examples).
But the other person doesn't want to the relationship to end or cause upset, so ends up trying to stick to these kinds of unreasonable demands.
I think in both scenarios the logic is the same, which is why it's such dangerous language.
The sole difference is the level of reasonableness (is that a word)
Basically if someone is a reasonable and loving person, they can have healthy boundaries with a spouse and they can both reasonably agree on those and respect each other.
If someone is controlling and abusive they will use the exact same language and logic but will use this against you with unreasonable demands and requests in an attempt to control you beyond what could be considered normal in a healthy relationship.