I suspect it's simply "one less thing to worry about when we sync with the cloud".

I suspect the long game here is eliminating user bookmarking altogether. Why would you want to use something as archaic as a saved URL, when there's a service that knows where you want to go to on the Internet better than you could ever decide for yourself?

They spent lots of money on Pocket and are increasingly pushing people to use it. Making the built-in bookmarks worse is an easy way to make Pocket look better by comparison.