I see your valid points. However, my point regarding backups being in a trust worthy area still stands. Idk why you would chance it by doing this. Besides that there are other reasons I will point out which I assume is their reasoning, statistically, is that Windows users tend to be a ton less savvy than Linux users, so they would be only backing up what is available on their system, and I bet on average they don't have more than 1TB drive with maybe 300gb if storage used that needs to be backed up, like pictures which is equivalent to the 1TB a month plan which I am assuming is the cost of the windows unlimited plan. If you want to screw over companies with exploits, please do so the evil/terrible companies; otherwise this makes you look like an asshole. My 2 cents, and no I don't work for them.

TL;DR - average windows user most likely uses no more than 300GB so offering an "unlimited plan" to them to make money on under-utilized plan makes business sense.