A lot of comments here are blaming the user for using the same username/password on multiple sites, allowing their account to be compromised. This is missing the point.
Nest cameras are a security product aimed at non-technical users, so of course this happens frequently, and of course this was foreseeable at the very earliest design stages. The designers needed to come up with a solution to this problem, and blaming the user is not an acceptable excuse.
The solution is actually pretty easy: require a pairing procedure that can't be done remotely. For example, the Nest app on a phone could display a QR code with is public key fingerprint, which you show to a camera, and the camera will only send video to phones it's been paired with. That would pretty much completely eliminate this failure mode.
Nest cameras are a security product aimed at non-technical users, so of course this happens frequently, and of course this was foreseeable at the very earliest design stages. The designers needed to come up with a solution to this problem, and blaming the user is not an acceptable excuse.
The solution is actually pretty easy: require a pairing procedure that can't be done remotely. For example, the Nest app on a phone could display a QR code with is public key fingerprint, which you show to a camera, and the camera will only send video to phones it's been paired with. That would pretty much completely eliminate this failure mode.