From my experience one of the biggest problems with streaks and smears on the insides is customers who clean themselves using traditional type glass cleaners which contain Isopropyl Alcohol, ammonia, solvents,etc.
Using glass cleaner on a regular basis leads to a build up of greasy residues left behind on the surface, using trad alone usually will not remove these residues properly and of course wfp can't be used on insides, the best solution i have found to remove these greasy residues is to scrub the glass with a magic eraser pad dampened with just water which will deep clean the surface and loosen all the greasy residues, then finish with trad just using warm water if possible without any additives or good quality microfiber cloths to achieve a smear free finish, just go into any pub or restaurant at night time and with the lights on it always shows up the greasy film thats left behind on the glass and mirror surfaces as they are usually cleaned on a regular basis with traditional type glass/mirror cleaners.
Just to share three types of traditional cleaners which are classed as CFU products are glass/mirror cleaner, furniture polish, and stainless steel polish. Tadgh