There's no movement roll, either, but you seemed to think that a valid comparison.
As to 'double-dipping' and defense rolls, the defense roll against a block is effectively whatever action that block blocks, so yes, there effectively is.
The movement comparison was a simple example of "You take some action, with shifts X. It is opposed by a block with shifts Y. Result is action with total shifts = X-Y".
That said, I can finally see where you're coming from, from a game-mechanical perspective - the difference in interpretation, then, is that I don't see a block as something you "defend against", while you see it as an attack that's defended by whatever action the target takes.
And, if you require the guy trying to counter-grapple to have a tag on an aspect (and not just a well-duh "I'm being grappled by an octopus" declaration*) - then I could see using your interpretation. Otherwise, it just makes the counter-grapple too powerful - the first guy spent an entire aspect tag, and all he gets out of it is to effectively give that free tag to an opponent? No.
*Footnote: if someone was so foolish as to declare such an aspect in a game I was running, I would laugh maniacally, ask "are you sure you want to do that?" and then, if they proceeded, offer a compel to have them removed from the scene as they get dragged into the depths - because, well, they're being grappled by an octopus. They just declared it, so it must be true, right?