1) I think a lot of start-ups want to hire "smart" people. Because they expect the new person to eventually wear many hats. Objective-C, Java, Android, CSS, server side concurrency, monitoring. An we've all seen Hunter and Schmidt reference that tokenadult usually posts when talk about interviewing comes around and it does seem that a general mental ability test (like an IQ test) combined with a work samples seem to predict future performance of that employee. Well except that one can't just straight up give IQ test to job applicants (there is a court case about that). So we are left with a job sample (which many forget to give, as is the point of the author). But instead many focus on the GMA and create proxies for it -- cute little puzzles about blenders, round manhole covers, and other such silly things.
2) Those interviewing don't know the technical stuff and are afraid you'd out-bullshit them. "How does an Ajax request work" well if the interviewer themselves doesn't quite know the details the might not be able to evaluate it properly. They could have it written down but well, some technical questions have many different levels of depth that a candidate might descent to. So a quick written answer to the question might seem wrong but it is really because the candidate is more advanced. So puzzles seems to be a generic and "easier" to handle.