Simply put, editorial and opinion sections are supposed to be like that. It's not an error; it's by design, and it's a design that has existed for as long as editorial and opinion sections have existed.

You, as the reader, are expected to be know enough about how newspapers are organized to tell that you're reading part of the opinion or editorial sections and what that means. In an ideal newspaper practicing journalistic integrity, letters to the editor like the one presented here would be published in future editions of the paper, or, failing that, published in competing newspapers.

Yes, everything is subject to bias. Everything is subject to bias. You cannot eliminate it. The point is to be aware of it's existence, and newspapers help do that by placing these types of pieces in their opinion and editorial sections.