For a person on the cusp, making a $600 a year may help. Now, erode that with the additional costs of filing (both financial and time) can make it no longer worthwhile.
The requirement is for Etsy to provide the 1099 to report income that the Etsy seller already should have been reporting on their income tax return (hopefully along with corresponding expenses to offset that income).
In fact, the whole point of this requirement is to make it easier for small sellers to properly report their income to the I.R.S., because now they don't have to dig through a year's worth of receipts to figure out how much they made from what source.
The alternative is that the person that goes and gets a W-2 job for $600 has a mechanism which forces them to pay taxes, but the person that does odd jobs and gets paid here and there via an app has no mechanism to force them to pay taxes. How is that fair?