You are thinking as a Canadian and as a buyer.
The GSP is sold to US (and now UK) sellers as a Seller Protection Program.
When a seller uses the GSP she only has to ship domestically with Confirmation of Delivery to the GSP plant in Kentucky. There her responsibility for shipping time, damage in transit, and delivery ends.
Those potential problems are accepted by PitneyBowes, who run the GSP.
The seller also cannot get negative feedback for import fees, which always have been the responsibility of the buyer.
For eBay, the number of US (and UK) sellers who will ship internationally has increased dramatically. This means more sales and higher prices on which eBay collects fees.
The only drawback for the seller is that Paypal charges an extra fee for processing international payments.
If the seller uses USPS /FCI, the buyer may still be charged duty and sales tax plus the $10 Canada Post service fee . More likely not, but it can happen and is just as legal as the GSP import fees. The seller may get negative feedback for this.
The mailed parcel may be slower to arrive than the GSP parcel, or slower than the buyer likes. The seller gets feedback on this.
The mailed item may be damaged in transit. This is usually the seller's poor packaging, but she gets the feedback for this too.
So -- if you were a nervous, provincial and xenophobic American, why would you NOT use the GSP?
