Wow. A restaurant in Chicago O’Hare, one of the biggest airports in the world, banned an infectious disease doctor from entering because they were wearing a mask. This is unacceptable.
You can file a complain about the restaurant at Chicago O'Hare denying service to mask wearers here: https://www.hmshost.com/about
@luckytran Thanks. I asked them the question on their contact form.
(BTW, I hate wearing a mask, but people should wear them if they want to or are around someone who prefers it.)

@luckytran They replied.
"Dear Sir/Madam,

We appreciate your email and wanted to follow-up.

Thank you for contacting us and giving us the opportunity to address this. We are currently investigating the matter. Our policy is to serve all travelers and we are committed to providing a safe and welcoming space for everyone.

Kind Regards,

The HMSHost Customer Care Team"