To me, the survivor riddle
hinges on the usage of language. By definition, a "survivor" is "a person who survives, especially a person remaining alive after an event in which others have died." The riddle uses misdirection by adding unnecessary detail (border of US and Canada) to obscure the fact that a survivor must be alive by definition and therefore does not need to be buried. If there are no survivors, then I guess mathematically the statement "All survivors were buried in the US" is vacuously true, but you would never say that in reality. The riddle should translate more closely to "Assuming that a crash occurred on the border between the US and Canada, and assuming that at least one occupant of the plane was alive after the crash occurred, where were all of the people who were alive after the crash occurred buried as a result of that crash?" (This last part eliminates the technically true answer of "Sweden, because Jimmy was on the plane and he survived, but then he lived another 50 years and died and was buried in Sweden").
The second problem preys on
the fact that most people have either (a) heard of the triangle area formula to find (6)(10)/2 = 30; or (b) heard of Pythagorean triples/triangles and know that there exists a 6-8-10 right triangle, so (6)(8)/2 = 24. People don't expect the fact that there is so much additional complexity behind this question - if I was asking you a quick math question about geometry, chances are you wouldn't expect to have to disprove the existence of the triangle in the question. There is an inherent simplicity bias as well here, since people want to give a quick answer without thinking too hard about it. My first thought certainly wasn't to prove that the maximum altitude of a triangle with hypotenuse 10 was 5.
I hope this helps - to your point, I certainly would agree with you that it isn't that big of a mistake. That said, in order to trick unsuspecting puzzlers, the puzzle relies on the puzzler making those specific mistakes in order to spring its trap, so to speak. So they are not big mistakes, but they are fundamental ones.