I think it's to stop people from posting just one long weak story to get a free account. If someone writes 2 stories that do passably well then it's likely they'll write 3 or more, while many people will write only one story and give up.
I've known so many 'authors' who write a single somewhat successful book, who never write another, despite their apparent success. It seems, if something comes too easy, it's just not worth the effort.
The writing process seems to thrive on struggle, on fighting the long odds. I must say, I've always had a natural inclination towards art and design. My brother, who struggled as a professional painter in Soho, marveled at my ability to instantly capture likenesses with pen and paper. But it case too easily for me, so I never too it seriously.
For me, writing was what I needed, because I struggle with every aspect of it, while things like cover design and interior graphic design is a breeze.
So, if this policy gets only a few authors to buckle down and crank out just one more story, it may be enough to encourage them to continue. If so, that's enough for me. However, if that policy doesn't have that effect, then it just seems bazare.