I’m assuming you’re referring to factory-installed privacy glass and not aftermarket window film. The answer has to do with a fascinating detail of federal law: SUVs (and minivans, for that matter) fall into the category of “multipurpose passenger vehicle”, which is usually regulated differently from the category “passenger car”.
In layman’s terms, the regulations pertaining to window visibility assume that the driver of a passenger car will have to look out of all of the windows in the course of normal driving, whereas an SUV driver will only need to use the front windows and windshield.
As a result, darkened privacy glass is permitted to be installed on SUVs, minivans, and whatever other vehicles that the manufacturer can somehow get the federal government to label as a multipurpose vehicle (including the Chevy HHR and Chrysler PT Cruiser, for example).
Since it’s obviously a desirable feature, the manufacturer is usually happy to make it when they’re allowed to.
You can stop reading here unless you’re interested in the specific regulation and technical details.
.
.
.
.
Here’s the applicable section of the Code of Federal Regulations (49 CFR §571.205). It doesn’t directly mention window tint, but look at S5.1 and S5.1.1: the ANSI/SAE Z26.1-1996 safety standard is incorporated by reference, and multipurpose passenger vehicles (MPVs) are required to conform to the window glazing* requirements for trucks.
I couldn’t find a copy of ANSI/SAE Z26.1-1996 that wasn’t paywalled; the closest I could do was this draft update of the standard from 2007. Since I have no reason to believe that the provisions changed from the 1996 standard, I’ve used it to construct this answer.
The relevant provision, stated several times in the standard, is that windows used in specific locations have to be at a level requisite for driving visibility, which requires those windows to pass Test 2 (luminous transmittance of at least 70%). The list of specific locations explicitly includes all windows on a passenger car, but excludes rear side windows and usually the rear window on trucks and buses.
*Basically, any part of the window except for the glass.