It's probably just a tactic to reduce resume spam. After being unemployed for awhile, people tend to apply to everything to increase their chances of success.
How hard is it to check the work chronology and just bong any resume that doesn't look like it would be anywhere near a good fit? It's not like H/R people are overworked filling hundreds of positions in their companies at the moment.
There's also a cost/benefit analysis. Having gone through a few hundred such applications, they have probably come to the conclusion that the likelihood of finding a gem is too low to be worth the effort. If you are employed, you are likely only sending out resumes if it's a very good fit; therefore, from the hiring HR guy's point of view, there's a lot of value in that self-selection.
I was talking to an accounting recruiter the other day. They're finally starting to have a decent flow of jobs in the pipeline, but he's being killed by the number of applicants... many of which merely had an accounting class or two in college on the way to their marketing degree.