Yes, it's pretty clear why they are doing it. But seriously? Down from $20K to $600? You know who that hurts? Not the "rich," whom we have been castigating for years for not paying their "fair share," that's for sure. No one who "sells" $600 worth of stuff a year is making any kind of bank. Hell, if your profits are $20,000 a year, you're barely getting by. Most of these people are just drumming up pocket money to turn around and spend on something else. It's a wash, and the items have already been taxed a bazillion times besides.
And what a paperwork nightmare. Not only will it be a burden to all these selling platforms, who now have to gather—and protect—sensitive information like SSNs and send out what could amount to hundreds of thousands of 1099s every year, but it will also be a hair-puller for accountants and tax preparers who will have to figure out how, exactly, to prove that those monies are not "profits," just the proceeds from selling Aunty Whomever's funky Christmas present that's been languishing in the hall closet.
Just stupid all the way around. Go after the real tax dodgers and leave the second hand dabblers alone.