Besides the few meters long extendable neck.

Exactly. She doesn't live with her neck continuously expanded. She looks completely normal AND has the ability to expand her neck at will. Like I originally said if you had quoted the whole sentence.

So why only those two treated her like a spectacle?

Only? I see very clearly in that panel that everyone was quite surprised by what she did because by all means it was a spectacle. She caused a spectacle. She did something cool, something unique that almost no one has the ability to do, out of nowhere, in front of everyone. I even see in that panel another girl taking a picture, who knows how many more did that Itsuki didn't see. The guys even took the picture because they were so amazed since they had never seen a (that they know, since they look like everyone else) a rokurokubi before. Again, was it a bodybuilder, doing some awesome feat of strength, would Itsuki be justified? Would it be so outrageous and "making a spectacle out of the bodybuilder" to warrant that reaction?

And, you know, these people here. Why were they trying to take a picture of her, if it's commonly accepted and known?

Also, it's not an ABILITY. It's who she IS. That's like saying a black guy made decided to make his color skin visible in the public by wearing a T-shirt and short shorts instead of covering himself in robes.

That is such a stupid question. The fact that it's accepted and known doesn't make it any less infrequent. Celebrities, people in traditional outfits of X country, sportspeople, those who can do stuff that others don't, they're accepted and known, that doesn't mean people won't try to snap a picture or be amazed at the rare occasion they see them and the fact that they do doesn't mean at all that they're marginalizing them or treating them as less than human.

"Who she is" is such a fallacious point. Literally the only difference between her and other people is that she can stretch her neck at will. Say, if I made up a "race" name for people whose only different to us is that they can fly, or that they have really long tongues, or that they're bodybuilders, does that automatically make it who they are and a taboo topic? How can you still be comparing her to a black person as if she was marginalized or they were comparable? Maybe if it was someone who has the power to change their skin color and they changed it in front of everyone then it'd be comparable. Funny how you try to circumvent into the more vague parts yet ignore the arguments that have you beat like the flightor bodybuilder one.

Your arguments make you sound like an SJW who doesn't realize their inherent racism and who sees xenophobia in every little thing.