I know why the GSP "wreaks havoc" for buyers and sellers.
Buyers are buying from GSP listings, not reading or understanding the terms and conditions, and crying about it later.
Sellers are using the GSP and are either unaware that they're opted in or else they haven't done their homework and researched how it looks for their potential buyers.
It's not rocket science.
And I thought my post was an even-handed explanation of the GSP, not a "defence".