Why would a company place rules which allowed them to claim your work if you used there services?
Plus I'm guess most cloud types companies are doing this now?
Are you paying for "their services"? Usually that's what them to the claim. If I let you borrow a pool pump and filter for free I think that entitles me to pee in your pool...If you rent/buy my pool filter/equipment then you don't owe me squat.
I paid for the damn account, give me my rights back, A******s! 
Is drop box free? Do you get anything extra for a paid account?
PS EDIT:
Eminent domain refers to a monetary reimbursement when Da Man wants your shight for "public use" As in roads or parks. Not to split hairs but they buy it from you, not really stealing. They Steal it from you when you refuse the money.
I think the author used a term when they simply should have said stealing, stole, ... maybe seize.