Why do I have to declare I’m either the Democratic or Republican Party on the “outside” of my mail-in ballot envelope? I believe that should be barred.

It's the law, King County Elections is just following it.

It actually is required by state law. U can confirm that with the Secretary of State's office. It was also requested by both of the major political parties. The information is public record and King county will keep the info for 30 or 60 days then discard it. Also, it doesn't hold you to that party forever. You can vote for any party you wish in upcoming elections.