The German titles of King novels are (almost?) always just a single word -- maybe the Dutch publisher is following that trend.
No, not at all. Of course the ones that originally are one word (like Carrie, Christine, Misery etc.) remain one word, as the ones that are people's names remain the same - Dolores Claiborne too actually.
Salem's Lot is changed to Bezeten Stad, which means Possessed City, which never sounded good to me, because possession seems more attributed to a person than a place.
Firestarter actually becomes three words: Ogen van Vuur, which means Eyes of Fire.
Needful Things becomes De Noodzaak, which is a wordplay, because 'nood' means 'need' en 'zaak' means 'shop', but together it's also an existing word meaning 'necessity'.
The Stand becomes De Beproeving, which means The Ordeal, as there isn't a good translation for 'a stand', I think.
Thinner could have been done literally, but becomes De Vervloeking, which means The Malediction (or Curse).
Pet Semetary could have been done literally, but they would have to find a way to do the spelling wrong too, and it became Dodenwake instead, which is Wake or Vigil.
The Tommyknockers is probably untranslatable, so it becomes De Gloed - The Glow.
Gerald's Game, like Needful Things, is done as a wordplay here: De Spelbreker, which means 'the spoilsport' (which refers to Gerald I guess, because he 'leaves' the 'game', although through no fault of his own), but it has the word 'spel' in it, which is 'game' from the original title.
Skeleton Crew is a wordplay too and can't be translated literally, so it becomes Duistere Krachten (Dark Powers), which is also used as the title for the story Word Processor of the Gods, which could have been translated literally.
Night Shift is named after one of its stories too, Children of the Corn, which could have been translated literally, but was changed to Satanskinderen, meaning Children of Satan. Again to make it more gripping, I think, corn in itself doesn't sound very scary or threatening. There is (or was) this tendency sometimes with translations of horrortitles from writers like King, Straub etc. to make them sound quite sensationalistic, like trashy novels or movies, and this is such a case.
Some are quite close, but with a slight alteration: Different Seasons becomes 4 Seizoenen (Four Seasons), Duma Key becomes just Duma, The Dark Half becomes De Duistere Kant which means The Dark Side rather than Half.
Then there are some that are literally the same, like Cell (Mobiel here), or, especially many of the later ones (and The Shining only of the earlier ones, although when it first came out it had a different title), where they keep the original English title: Desperation, Dr. Sleep (dr. rather than doctor here), Joyland, Revival, Mr. Mercedes.
Also nowadays I think more people are able to understand English here, because of things like pc's becoming more general, than when his books first came out.
But in the end, it's hard to find a solid reasoning behind the translations of the titles, it's just what the publisher liked most at the time it seems.