Although the responses above seem to address the original question well, there is another angle to the question that I feel should be addressed. Built in to the original question is the question:
"How in the world can anyone look at the first two moves of the game and say so much about how the rest of the game will proceed? Can't anyone play any move they want?"
My response goes like this:
In the broadest sense, NO - nobody can say anything about how the rest of the game will proceed. For all we know, white will respond to the French with 3. a3 ... 4. a4 ... 5. a5 ... followed by 6. Ra2 .. 7. Ra3 and so forth. After which I'm sure black could really open up the position.
Discussions about opening theory don't consider absurdity like this - they assume that if you are even interested in the topic you are already beyond making pointless moves.
When opening theorists state things like "the French defense leads to imbalanced positions, strong pawn structures for black, etc. what they really should be saying is "assuming competent play by both sides, the French leads to (insert whatever here). Similar to dfan above, authors grow tired of writing "assuming competent moves on both sides" in front of every sentence they write.
In the case of the French defense, white really only has two or three viable options, after which black only has two or three competent responses, and so on (check any opening table or discussion of the French defense to see what they are). Besides, many of these responses ultimately lead to the same position anyway. The fact that for the first ten or so moves, most opening moves only have two or three legitimate responses, two or three counter-responses, and so forth leads us to make statements like "the French leads to closed positions and strong pawn structures."
Just thought I'd clear this up for the original question and any future readers of this post.