Because we want to design decision theories that are resistant to being blackmailed, but that can get involved in negotiations. If there's no meaningful difference between the two...
Can we just use that as the definition?
Define a pre-commitment by you to be blackmail if it makes me wish that I'd pre-pre-committed (and, of course, let you know that I'd pre-pre-committed) to not do the thing that you want in the event that you made that pre-commitment.
How does that do?
EDIT: Thinking about it more, this problem is just division of gains from trade. I'll explain that more in a top-level comment.
For a more parable-ic version of this, see here.
Suppose I make a precommitment P to take action X unless you take action Y. Action X is not in my interest: I wouldn't do it if I knew you'd never take action Y. You would want me to not precommit to P.
Is this blackmail? Suppose we've been having a steamy affair together, and I have the letters to prove it. It would be bad for both of these if they were published. Then X={Publish the letters} and Y={You pay me money} is textbook blackmail.
But suppose I own a MacGuffin that you want (I value it at £9). If X={Reject any offer} and Y={You offer more than £10}, is this still blackmail? Formally, it looks the same.
What about if I bought the MacGuffin for £500 and you value it at £1000? This makes no difference to the formal structure of the scenario. Then my behaviour feels utterly reasonable, rather than vicious and blackmail-ly.
What is the meaningful difference between the two scenarios? I can't really formalise it.