[grey-area]

Bob wants to check his email on Alice's computer, so he logs Alice off and logs into to his account. This syncs across all website he visits...make sure no one else can accidentally (or intentionally) siphon away your browsing history.

Don't do that then.

There's a simple solution to this conundrum, but it involves google not hoovering up the browsing history of half the world by default. Unsurprisingly, the google team has decided not to implement that solution and instead has tied logins on a web page to logins in a browser ever more tightly, and this move is just another step on that road, until you can tell yourself that 99% of the world logs in to the browser, because it's just easier, so we'll make it opt out instead, and then by a series of small incremental steps, each of which seems reasonable, you're forcing users to log into google and send them your data to get any browsing done at all.

Logging in to the browser is the problem here, not the solution. You should log in to websites, not the browser, that separation is a good one and is there for very good reasons.