Sellers are not "forced" to use the Global Shipping Program.
It is heavily promoted as a method of avoiding bad feedback from foreign buyers whose postal systems do not work very well (as well as those few who are outright scammers).
Unhappy foreign buyers, who do not recieve their items promptly, do (and should) open Item Not Received disputes and win refunds before the item arrives. This hurts the seller.
And Americans are notoriously inward looking. A seller might shrug off a "undelivered" claim from California, but go ballistic if a similar claim came in from an Australian whose purchase was shipped by Surface Mail.
In addition, eBay is on a bit of a sales plateau, and one way of increasing those sales (and getting more fees) is to encourage xenophobic US sellers to sell internationally.
But the program does not work well for purchases valued under $50-$100. It does not work well for Canadian buyers.
And it does not work well if there is a problem with the item purchased. There have also been some reports of parcels being opened (which is probably done under the supervision of CBSA. Probably.) and then badly repacked causing damage.
Personally, I avoid sellers using the Program. But there are some advantages to it on high-price/ high-value/scarce items which might not otherwise be available to Canadian buyers.
