The specific policies relating to in-flight entertainment are a matter for individual airlines. They will tailor content to the demographic of their typical passengers.
Air crashes and similar themes are almost universally avoided, but there are significant regional differences leading to films being more or less heavily censored, or simply not offered.
For example - airlines based in Muslim countries will censor references to pork and pork products, either by deleting the scenes, or editing the dialog, or both.
Europeans are tolerant of some nudity, but dislike excessive violence, while the Middle East will censor all sexual language and bare skin, but permit a much higher level of blood and gore.
Other, lower-level censorship may also occur. Logos of competing airlines can be pixelated or deleted.
This is important to airlines. In February 2013 a United Airlines flight was diverted because of a 'passenger incident' originating when parents complained about 'inappropriate content' for their young children (http://www.telegraph.co.uk/travel/travelnews/9973766/Sexually-explicit-in-flight-film-causes-plane-diversion.html)
Much of this information was sourced from CNN. Apply your own judgement when assessing the veracity of such a source.