Not sure what kind of diffusion they use on csi. But, there are many reasons for using filtration in front of the lens. Many are personal choices. For one thing, you can determine what the the diffusion will look like in your frame at that time. What if you were unable to get to post session and your editor was free to add his own diffusion. I personally don't think it is the same electronically. but, i'm sure it is becoming more and more transparent.
It's far more difficult for a computer to look at a raster of pixels and determine what things need diffusing...it becomes a pretty tedious manual setup.
Various types of optical diffusion (camera filters like the ones you mention) have varying effect on objects that vary in focal distance, fine texture, or exposure level. For example...Black Pro Mist can remove facial skin "ruddyness" if you will, but leave high contrast edges (specular highlights or an object's hard edge) relatively sharp. It takes a lot of computing power for a software program to figure out whether a fine detail is a skin blemish or a specular reflection on an eyeball...and even with an application that can do it, I'm still not sure I'd trust it to be automatic...
Bottom line is that an in-camera effect takes the time to put the filter in the matte box, doing it in post requires a LOT of work and most likely rendering with the current technology...
I think the show is shot on film, as you might know, DPs have for 30 years begged Kodak to make their film stock softer, with less contrast etc. Fuji had been typically softer or less contrast so Kodak has had an incentive to give the DPs what they want. I sense the ones that ask for the lower contrast film are the ones who cant seem to get control of the tools outside the film stock so they ask someone else to solve their lack of skill. Interestingly, DPs who shoot on 16 have been super happy with the contrast on their Kodak film stock, so, Kodak, without making it public knowledge has been making one stock for 35 and one for 16 but talking about the matching stocks as if they were the same. Took me years to figure this out and finally a Kodak rep admitted it at one of their in house shows.
DPs often use diffusion to cut this contrast but it is often done in a less then subtle way. The first kind that was popular, and is still used is to stretch one layer of a woman