I have encountered this before, and it always struck me as a little unfair, but I couldn't ever think of a reason why it wouldn't be permitted. These types of rules go by several names, sometimes "spousal carve-out," or "working spouse surcharge," or "spousal exclusion," and they can take many forms. Sometimes, the health plan will totally exclude spouses from coverage under the plan if they have healthcare available with their own employers, and sometimes they just charge more. The federal law that controls most employee benefits, ERISA, doesn't say anything about this situation, so there's no reason to think it's illegal, even if it does seem like it should be. Generally, employers have a lot of latitude in crafting their employee benefit rules, so things like this are probably OK, unless and until a law changes.
There's an article from the Society for Human Resource Management (SHRM), the professional organization to which most HR people belong, which talks about this more. You may want to read it, as well.
To read the article, just Google the phrase "shrm spousal exclusions on the rise" and click the first link. (For some reason, if I try to link people to this article directly, the website says it requires a subscription, but if you get there from Google, you can read it for free).
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