This is only a guess, but my understanding is that Mercedes has to approve the designs that Sprinter conversion companies use and sign off on the structural integrity, appearance, etc before they allow them to sell them as new vehicles with the badge on the grill.
I believe the factory Sprinter trucks have no operable windows aft of the cabs. But apparently Mercedes approves the small tilt-out windows some converters use. Ours came with screens.
They actually work fairly well because in addition to airflow with the roof vent & F. Fan, the little windows can be used when it is raining, and can be left open when you are away from the RV or while driving. It would be hard for a thief to reach in and get anything or gain access through them. We leave ours open during hot weather while parked in our driveway to let heat escape.
You can buy or make screens for your front cab power windows very easily - we bought Skeeter Beaters online that attach around the driver & passenger window openings with magnets.
Some newer conversions have very nice optional or standard screen arrangements inside of the side sliding door and at the back, inside the two cargo doors. Some Sprinter suppliers sell screening for these openings, and some peope make their own.
We rarely use the AC unless the humidity is really bad.
