Venom's PG-13 rating may have been decided to keep the possibility of a crossover with the MCU's Spider-Man (Tom Holland) alive. Sony may be working with Marvel Studios as they reboot Spider-Man with Tom Holland, but they've still got their own plans. Spider-Man: Homecoming being a giant hit only further made the studio interested in building off of the revitalized interest in Spidey on the big screen, so they got Tom Hardy to star in Venom - a movie that is currently not part of the Marvel Cinematic Universe.
There's been contradictory reports over whether or not Venom will connect itself to Holland's version of Spider-Man, and the MCU as a result. Rumors claimed Holland filmed a short cameo for the movie, but there's no confirmation (or denial) that this is the case. With Spider-Man's involvement unclear, many hoped Sony would unleash Venom with an R-rating. That won't be how he's portrayed in the solo movie, and it may be because of Spider-Man.
Fandango's Erik Davis shared what he's heard in regards to why Sony wanted Venom to be PG-13 instead of R. It is very simple: They want to have the option for Spider-Man and Venom to interact on the big screen in the future. Here's what Davis revealed on Twitter about what his studio sources are saying, "The reason is so that Venom and Spider-Man can face off at some point down the line. If the movie is R, it’s hard to do that."
The official PG-13 rating for Venom may not be what everyone wanted, but it is what Sony's aimed for. It was previously reported that they wanted a PG-13 rating for Venom instead of R, although they still wanted to "push the limits" of PG-13. This has all been connected to their hopes of latching Venom and possibly the rest of their planned spinoffs to the MCU. Even if Marvel Studios head Kevin Feige doesn't acknowledge their work, Sony does still own the rights to Spider-Man (and is simply sharing them with Marvel Studios for the time being), so they can and likely will do as they please.
If Venom's PG-13 rating was mandated for the hopes of a larger universe, it obviously begs the question whether or not this is the right decision to make. While Venom's box office projections look great, those numbers could change if reviews for the film are negative and point to a watered down version of the character. Sony's been guilty of putting a cinematic universe in front of a single film before, so hopefully they've learned their lesson before. If a PG-13 Venom is what is best for the character and the film, then the possibility to crossover with Spider-Man down the road is an added bonus. But, if that crossover potential is the driving force behind the rating and is actually doing a disservice to the symbiote anti-hero, then they will never get the chance to fulfill those dreams. We'll know soon enough whether or not Venom is a hit and what chances Eddie Brock has of meeting Peter Parker in the future.
Source: Erik Davis