I don't think I've mentioned the one logical fallacy that is perhaps the most lethal to accurate interpretation: making an unwarranted assumption.
As an example, I excerpt a comment I made on JP Manzi's Return of the Prodigal Blogger:
Why would anyone assume that when 1 Corinthians 15:22 says, ""For as in Adam all die, even so in Christ shall all be made alive" that it means God will save everyone, whether they accept or reject His Son, no matter what? Even in the face of dozens or hundreds of scripture passages to the contrary? How can you separate that scripture from its context? Above it is a mention of those who have fallen asleep in Christ. Below it is a mention of those who are Christ's enemies, to be put under His feet.
Paul even negates this logical fallacy in those verses:
The last enemy to be destroyed is death. For he "has put everything under his feet." Now when it says that "everything" has been put under him, it is clear that this does not include God himself, who put everything under Christ. When he has done this, then the Son himself will be made subject to him who put everything under him, so that God may be all in all.
There are times when "everything" doesn't literally mean "everything." There are times when "all" doesn't literally mean "all."
Here, "all" must be interpreted to mean "all believers" (to whom Paul was writing; not to unbelievers) in order to be consistent with other scripture.
Paul's words address a heresy that no resurrection occurs; not a belief that God saves everyone, so one must be cautious about making them say something more - or different - than they were intended to say.
In the end, I think the answer to the question "Why?" above is that people will make unwarranted assumptions - and other grave errors in scriptural interpretation - because they desperately want to reach and believe the conclusion to which they feel it leads. Even if it means leaving out the phrase "by faith in His blood" in Romans 3:25-26 and all those other scriptures which weigh in heavily against it.
In the end, the answer is intellectual dishonesty and arrogant presumption.