Gonna make some generalizations here:
Most HR people are lazy, so any excuse to cut down on resumes is good enough for them. If you give HR too much power and not enough oversight, then you get policies like this that are designed to make HR staffers jobs easier.
Most companies of significant size digitize resumes and make them searchable and store them in a database. So, extra resumes from the hopeless shouldn't be an issue.
Having said that, as an employer, the employed are always are better bet than the unemployed. If someone is willing to pay Candidate A, then Candidate A is justifying his/her existence somewhere. If no one has been paying Candidate B for a while, then how do we know he is good at what he claims is his area competence?
Now, I am sure there are diamonds mixed with sand in that big bucket of the unemployed, but it's harder to fish out the diamonds, and all the sand are shouting that they are diamonds that just haven't been polished right.... It's a lot easier to hire a cog that is currently working as a cog at a competitor that appears to be working like a clock.