Can airlines deny boarding if my port of entry is different than the country which issued the Schengen visa?

Few days back I did a Europe tour from India. As stated by many (I haven't confirmed official documents), I can apply for a visa from the country which is either my first port of entry in this case Paris or the longest stay which in this case was Italy.

Now following is what happened with me.

My flight was New Delhi to Paris via Istanbul with Turkish Airlines
I Traveled from New Delhi to Istanbul (no problem here).
When I was boarding to flight Istanbul to Paris, zhe airlines officials (the ones who checks the boarding pass before boarding the flight) told me that I could not board the flight because my visa was issued by Italy and I need to go to Italy First. They were denying the longest stay option of the visa.
Moreover NONE of the staff of Istanbul airport were able to understand English so I had to go to a lot of problems before I managed to get tickets to Venice.
I somehow managed to fly to Venice from Istanbul after almost 24 hours of struggle at Istanbul airport (without food and water).

So, my question is do the airlines staff have a right to deny flight boarding in this case?

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Do I have to enter the country that issued my Schengen visa?

It does not need to be the country that issued you with a Schengen Visa, as you must apply with the nation where you are planning to spend the majority of your stay in the Schengen Area.

Is there passport control between Schengen countries?

The Schengen area comprises 26 European countries. There is normally no passport control on the borders between these countries. If you have a valid visa or residence permit in one of these countries, you can also visit the other Schengen countries.

Can I change my itinerary after getting Schengen visa?

"can i change my itinerary within the schengen area ?" You should follow the itinerary you gave when you applied for the visa. You cannot change your itinerary so completely that the information you gave in the visa application is no longer true.

Do embassies know about visa refusal in other countries?

As long as there is no visa rejection stamp on your passport none will know that another country has denied your visa. Except for Schengen countries as they use the same system and they will be able to pull you information. In the Western world all of the embassies are linked into common computer networks and systems.

THE SCHENGEN ZONE TRAVEL EXPLAINED - DIGITAL NOMAD TV

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

Images: Karolina Grabowska, Karolina Grabowska, Felicity Tai, Rachel Claire