Why is a healthcare insurance provider collecting income information on the people they insure?
I've worked in employee benefits for over 25 years, and the usual reason is that they are administering more than your health insurance. Often you also have short-term and/or long-term disability insurance, or life insurance. The benefits of these are based on some percentage of your salary. Your short term disability benefit may be 60% of your salary, or your life insurance benefit may be 2 X salary.
In all my time working for insurers like Anthem I have never been asked to pull salary data for anything not related to the above.