I have been aware of this for a few years. While the "Free CarFax available" is definitely valuable, I find the "CarFax 1-owner" badge displayed in the SRPs is even more valuable on Autotrader.com:
I deleted the dealer data in these listings, but you can see that the dealer who has CarFax enabled gets an extra "1-owner badge." In SRPs, I believe conveying that your vehicle is a 1-owner is helpful in addition to photos, price and comments. On Cars.com, the difference is much smaller as it says, "CARFAX Record Check" versus, "Free CARFAX Report."
Now for our dealership's side of the story:
We are part of a multi-store group that shares the same owner. However, all of our dealerships are managed completely independently. I contacted CarFax to sign up for the unlimited CarFax package and was told that
if we didn't sign up the entire group, they would not allow us to sign up for unlimited CarFax program. Apparently, corporate CarFax has decided from the top down that if you are part of a group, it is all or nothing. They said that this was due to some dealers only signing up one dealership and sharing the password for the whole group. I understand where they are coming from, but I feel that they could figure out a better workaround (i.e. IP filtering).
At any rate, we use Autocheck and other than the SRP badge, I haven't noticed a difference. In fact, I put "Vehicle history reports are available for all vehicles at our website" in all of my comments.