California is also going GDPR-like

The California Consumer Privacy Act of 2018 ballot initiative could give consumers the right to ask businesses what personal data is being collected about them and how it’s being used.
[mediapost.com...]

and like GDPR no matter if you are US business or not

The act will apply to every company that uses data from residents of California

Another reason, when you make your site GDPR compliant, to apply the same rules to all visitors and not only EU ones...