What he is saying is that every spec has a specific purpose and idea. If a combat rogue is designed around the idea of using a fast offhand, then it should be clear that you should be using a fast offhand. That is all he is saying. They don't want people playing combat rogues to pick up a slow offhand and then wonder why their dps is so low or why certain things aren't working, it should be obvious. This change will probably be no more than putting in a tooltip or something remarking on the fact that they should have a fast offhand.

To address the point as a whole, if every spec had as much freedom as you seem to want, then what is the point of having specs? Of course the designers want you to play the game a certain way, that is why they made the game. If you could do whatever you want whenever and however you want, it defeats the purpose. He is not saying that they will remove any freedoms, just putting it all into perspective, play how you want to an extent, but realize that their are limits to what you can do if you stray too far from the intended design of your class. You don't take a sports car off-roading, they aren't meant to do it, but that doesn't mean you can't and no one is stopping you, it's just not optimal.

The wording might sound a little harsh to you, but it is very clear as to what he is saying.