If you happen to have kept your Coding Edge from June, 2006, I authored an article on this very practice. One of things I mentioned was that those coding assessments should be valid and reliable.
You mention that "tricky" questions are asked; indeed, some questions on the pre-employment exams are not appropriate and may not even have an accurate answer key--so always keep that in mind.
What's discouraging with the vast majority I have taken in the past is the tendency to put questions on them that are outdated, have no recognized coding reference (e.g., is a matter of opinion) or largely contain obscure information. I also find employers rarely update or alter these--they stay the same year in and year out. That certainly is not reflective of our coding manuals and guidelines and should not stand on our assessments either!
As for why this is done, there are certainly practical reasons behind it. However, all purpose of the exam is usually undone by its lack of validity and reliability. In my own opinion, the long assessments are neither necessary nor reflective of job duty, simply some manager's supposed need to objectify coding skill.
Good luck to you and just keep trying with them. Sooner or later you'll encounter one that makes sense and represents your skill level.
Kevin B. Shields, RHIT, CPCO, CCS, CPC, COC, CCS-P, CPC-P, CPC-I