WD introduces new, NAS-Specific Hard Drive line

I don't think there's any contradiction between the pages.

In the product page they pitch the fact that the Red drives are factory balanced in order not to create rotational vibration, and thus being specifically featured for multi-drive environments. Lower in the page they even state in big letters:

Ideal for. Specifically designed and tested for small office and home office, 1-5 bay NAS systems.

Then in the FAQ they state that Green, Blue and Black should not be used in a NAS with more than 2 drives.
Lower in the FAQ they also state that Green, Blue, Black and Red should not be used in Enterprise Grade NASes using Enterprise HBAs and for Critical enterprise applications. So, if you are running a NAS in a Bank, Airport, Hospital or any other critical activity you should prefer the RAID-Enterprise (RE) line of drives as they have time-limited error recovery (TLER), are tested extensively in 24x7 RAID applications, and include features like enhanced RAFF technology and thermal extended burn-in testing and the kitchen sink (well, that part about the kitchen sink may not be in WD's page 8) ).

So, basically, joining all the info together you get:

You have a Home/Small Office NAS up to 5 bays -> Caviar Red
You have an Enterprise/Critical Operation NAS -> RE4