They actually use a public macOS API for those Finder icons. It seems like there's no reason at all they needed to do this. It's really shitty.
Fortunately their service is now a commodity (as Jobs told them when they demoed for him, they are simply a feature). I have always preferred Drobox just because it seemed the simplest (and they supported Linux). But switching just takes a short time to copy my files to one of their competitors.
I prepaid for a year's Dropbox service. I wonder how much luck I will have getting a refund on the unused amount.
I wonder why they chose to gratuitously be assholes?