In some countries this information turns up when you do your taxes for the end of the year. And not all job markets are as huge and "anonymous" as the U.S. job market - some (like Israel) are relatively small (a few 10ks of programmers probably) and word gets around. Besides, if you get asked on an interview about what you've done in the recent past in terms of looking for work you're basically lying if you're not mentioning a brief period of employment. Not a huge deal, but still it's not honest and some people would rather not.