With Goons, every time you buy something, you get +1 VP.
With Swamp Hag, every time an opponent buys something, they get -1 VP and a junk card.
Without trashing, SH effectively neutralizes any point gain Goons would get, and fills their deck with junk, especially since a Goons player wants to buy more than one thing. Having more than one Goons in play can counteract this, but you can also have more than one SH in play. Once all the Curses are gone, this goes away, but if the Goons player has all the Curses, they're not going to be buying much.
Wish I could actually test this out. :/