The option seems to have been enabled by default at least on some Google accounts.

It would be interesting to know why the option is enabled by default for some (most?) Google accounts and why it isn't for some (few?) Google accounts.

If that checkbox was already checked when you got to that page, it’s opt-out, not opt-in.

The checkbox was not already checked, i.e., it's opt-in for me. But it seems that for most users, it's opt-out.

I agree, however, that my screenshots shows the checkbox checked. The reason is that I want to mark the line for clarity. By double-clicking the line in Chrome, the checkbox got checked.

Without the line marked, the checkbox was unchecked:

http://i.imgur.com/cas6SMB.png