It's pure corporate greed of course.
I'll go further by saying that Prime is unnecessary in and of itself. It's just an invitation to buy always more stuff. I buy stuff whenever I feel like buying stuff and not because I need to use the advantages of Prime. On Amazon France if your purchase is of 25 euros or more, shipping is free so all I need to do is wait till I need to buy enough stuff so the total is 25 euros or more and I have no shipping fees. why would I throw 50 euros away in Prime when I can perfectly do without it?
Also Prime over here is 49 euros/year, when you say 100 and now 120 Dollars, do you mean per year? Cause for once something in America is super more expensive than here in Europe.