Refused boarding pass for a return flight due to passport expiry in less than 6 months [closed]

I am an EU national married to a US citizen with a son under 2 years old, who has dual citizenship (my home EU country + US). We all reside in Switzerland, which is not part of EEA, but is part of Schengen, and have valid residency permits (which for my spouse functions also as a Schengen visa as far as I understand).

Our son's American passport is a 'temporary' 1-year old passport. He does not have an EU passport yet.

Recently my spouse and son went to visit family in the US while I stayed home. They flew directly from Switzerland and had no issues boarding the inbound flight, even though my son passport's validity expires in less than 6 months from the return date.

During their return trip 2 weeks later, the airline's check-in desk staff refused to issue boarding passes, citing my son's passport expiry date as the reason. They suggested having a new passport issued (which due to my absence would not be possible). Eventually due to my spouse's persistence they let them on board, but I would like to know if the airline's stance was justified, as I already started having thoughts of my family being stuck in an administrative limbo until I flew to the US to have my son's passport renewed (where I can also be denied entry to make matters more interesting).

The question I have is:

Is it true that in this case (return flight to the country of residency) the airline could get fined ~$20k or was it just ignorance of the airline's staff, applying a blanket US -> Schengen travel rule?

If they were right, what is the legal framework for such denial of boarding (I guess it must be entry rules for Switzerland/Schengen)?

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Refused boarding pass for a return flight due to passport expiry in less than 6 months [closed] - Stylish man with suitcase and passport walking along airport corridor

Refused boarding pass for a return flight due to passport expiry in less than 6 months [closed] - Cheerful business people with passport in airport hall

Refused boarding pass for a return flight due to passport expiry in less than 6 months [closed] - Orange and Green Label Airplane Ticket

What happens if your passport is not valid for 6 months?

Most countries will not permit a traveler to enter their country unless the passport is set to expire at least six months after the final day of travel. That means if your passport has less than six months remaining until the expiration date, you should renew it right away.

Does my passport have to be valid for 6 months?

As a general rule, passports must be valid for six months beyond the date the traveler will exit the United States. However, the United States has signed agreements with a number of countries to waive this requirement.

Can I travel if my passport expires in 20 days?

Answer: Your passport is valid until the date of expiration. The only problem may be the entry requirements of the country or countries you plan to visit. Many countries require your passport have 3 to 6 months validity remaining before they will allow you to enter or issue a travel visa.

6 Month Passport Validity Rule | Can I travel with less than 6 months left on my passport?

Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.

Images: Gustavo Fring, Gustavo Fring, Gustavo Fring, Torsten Dettlaff