Because occasionally you get strange trailer placement.
I used to work in trailer placement. I remember a ‘scandal’ from back in the day. The film Fly Away Home the film company decided to attach the trailer for The Mirror has Two Faces. Technically OK because it was a ‘green band’ trailer but since Fly Away Home was about a 12 year old girl and had lots of 12 year old girls in the audience with their moms, well the trailer got yanked by the end of the weekend.
I thought I remembered seeing an ‘R’ rated trailer at least once ages ago, but can’t ever recall seeing one again. Started to think I dreamt it ever even existed…
Red band trailers were kind of a big thing a couple years ago…seemed like the only way R rated movies were being promoted was ‘we have a red band trailer, come see it at…’
I saw "Shame"in a theater. It was rated NC-17. The preview had the “this feature” text. I can’t remember what the film was but it showed nudity in the trailer.
I’d always assumed that the “all audiences” meant that the stuff that triggered the R rating wouldn’t be in the trailer shown to a PG audience.