Overlooking the (usually newbie) sellers who were signed up by eBay without their knowledge, the program is designed to PROTECT SELLERS against unwarranted claims of non-delivery (Item Not Received) and negative feedback from foreign buyers who did not know their country charges duty and/or sales taxes on imported merchandise.
If you buy from a US seller who has GST enabled, he can cheaply send the item to PitneyBowes/GSP in Kentucky with Confirmation of Delivery which cost 25c or less.
And there his responsibility for delivery ends.
All applicable duty and taxes have already been paid by the buyer. If the buyer wants to complain about it, too late. And eBay will remove any negative feedback mentioning import costs, protecting the seller.
If the item is damaged in transit, remediation* falls to PB/GSP not to the seller.
PB/GSP tracks the parcel at no cost to the seller (the buyer pays this of course). This further prevents false INR claims.
EBay does mention in passing that it does not recommend using GSP for items valued under $50USD (around $65 CDN more than triple the allowance for duty-free imports into Canada.). But they don't remind sellers and as mentioned many sellers do not know they are signed up until they get a winning bid from a foreigner.
EBay started the GSP because sales in the USA were flat and US sellers, a xenophobic lot, were terrified of shipping outside the Lower 48. Many will not ship to Puerto Rico or Hawai'i, nevermind Canada.
From the point of view of making more items available internationally, the program has been very successful.
*And if your item is damaged in transit, PB/GST don't demand return for refund. Which saves the unhappy buyer a lot of money since international tracking is expensive.
