There isn't a reason at all?
Yes there is; to increase profit margins on items that are already razor thin.
Regardless if it is called FSSS it's not free for Amazon. They have to make up the shipping costs somewhere. I've received many items from them that literally must have cost more to ship to me then I paid---even with logistically sound shipping hubs.
Why should I as a customer want to return profit margin to them? I'm only interested in finding the best deal. It's irrelevant to me whether the item ultimately sells for a profit or loss, I only care about MY bottom line. Not Amazon.coms or anyone else. I go where the best price is. Period. Which is why I buy blu-rays from anywhere (but same applies to games or anything else). I will only buy during sales, or clearances, or if I have a coupon. Doesn't matter whether it's Fry's.com. Deepdiscount.com. ImportCDS.com. BestBuy.com. Target. ANYWHERE. I will buy from any legitimate retailer at any time if there is an item I want for the right price, there is no reason at all for me to be loyal to just Amazon.
Obviously there are realities with shipping. It is not a mystery why Best Buy could not continue to do free shipping with no minimum as they had for a couple of years. I voiced my displeasure with that, but obviously I've known that would inevitably be temporary (my attitude with that one was more of a "it was nice while it lasted", and believe me it was very nice). With Amazon, they've been $25 for as long as I can remember, like decades, so moving to $35 is sort of a big deal.