New boards should be private by default.
This includes the 'My First Board' board that is automatically created for new users — given its name, it is not unreasonable to think that eager new users might add bookmarks here without realising that it is publicly visible; the padlock icon (of lack thereof) is easily overlooked.
Users should have to explicitly consent to sharing their data by opting-in, not out. That this is not currently the case sends a poor message in terms of how your company views issues of privacy, regardless of your actual stance. As a new user myself, I found it very off-putting and my trust in the service has been diminished.
Randy. What was the result of the ticket you created?
Landice Fu commented
I agree with psub. Maybe you can create a preference for the default behavior.
Thanks you for showing your concerns about privacy! I'll go ahead and create a ticket with our developers about this issue. Thanks again for expressing how you feel and have a good rest of your week!