For one thing they have completely different privacy policies. By signing into the chrome they are automatically forcing people to accept the privacy policy that is far less private than the when people aren't signed in.
> The personal information that Chrome stores won't be sent to Google unless you choose to store that data in your Google Account by signing in to Chrome.
That doesn't say "unless you enable sync", it says it changes "by signing in to Chrome". Since they're now forcing you to sign into chrome without your consent they are also forcing you to accept that they can send your data to their services without needing any additional permissions from you.
> Signing in enables Chrome’s synchronization feature.
This is no longer true, so it seems most likely that that entire paragraph is simply out of date.