Because, barring the 500 for the weekly, those people using only the matchmaking service aka LFR/LFD would otherwise only be able to get 700 per week.
It is NOT the raid that gets you the valor, it is finishing the objectives of the matchmaking service that awards it to you. Since normal+ don't have that, they obviously don't reward it.
That's what I got from some blue post that also basically says that additional tanks/healers running LFR is a nice sideeffect.