I believe it's not just tax free, right, it's just priced into your salary. They won't give you the employer contribution back as cash if you decline, will they?
But it's better/worse than that. Because they won't pay you $55k if you decline healthcare. Instead, you get paid $50k with the option to get healthcare (take it or leave it).
My family's insurance healthcare isn't tax-free, which would mean saving $2k or $3k or whatever. It's over $10k of insurance a year that I pay less than $2k for.