Hindi treats "rice" (and all grains, noodles, flour, etc.) as a mass noun. It always takes the plural form. (Someone offered an excellent explanation in an earlier thread which I'll try and find and repost here...)
Just to try it, I added an answer without 'hain' (because of the 'nahin'), but it was marked as wrong. Is there something special in this negative sentence and therefore you need the 'hain', or should it have been accepted?
I have no idea either. The only situation I can think of when this form would be used is when one is talking about rice grains or more than one sample of rice grains. But that still doesn't answer why they have used the plural form as a translation of "The rice is not good".