Great question — answering as the UphillTech team:
Movie studios themselves provide the edited versions. They do the overdubs of foul language, and in some instances provide alternate scenes to ensure the movie still runs close to the original length (rounded to half hours with commercials). One of the most famous and sought-after examples is TV broadcast versions of Fast Times At Ridgemont High, which used to include 18 new or extended scenes not currently found on any official DVD release. Amy Heckerling, the Director, has said she hated the scenes so much that she will never release them, and with newer and less restrictive laws for what you can and can't show on Cable TV, the only way you can find those scenes is on a VHS tape someone...
Full answer by Johnny Bones: https://movies.stackexchange.com/a/92654 (CC BY-SA 4.0).
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