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Some years ago (2014) my friends and me did a trip to New York City, in which we were having meals in many restaurants and giving tips to the waiters, the way like it's done in the USA.

However, later on, we visited a popular skyscrapper building and we paid the tickets in cash to the woman who was selling those. When she was taking the money she said something like: "Ok, you're six people, it's 120$ in total and I'm going to grab 15$ in tip.".

Even if I didn't realize about it at the very beginning, I felt annoyed later on when we were already in the lift. In my country it's not usual to give tips and we do it to show gratitude when the service in some restaurant goes beyond the standard. I understand that in the USA it works in a different way, but I think you must do some kind of extra effort appart from grabbing some tickets for the customers. This situation isn't into the table given in this answer.

Is this situation common? Or did the woman take advantage of us for being foreigners?

You were taken advantage of.

Tipping is very prevalent in the US, but it is extremely unusual to tip someone who sells you tickets to a tourist attraction. I've never done it or heard of it. Even if you were being given a guided tour, where tipping the guide is normal, its usual to do it after the tour and after you have decided how good it was. The cashier took advantage of your lack of knowledge of what is proper.

To expand the question a bit, for any service representative to decide what tip they want and take it is extremely bad manners, and stops short of theft only if you are given an opportunity to say no. The only exception is when a 'service charge' is advertised as part of the price.

If you are in this situation again with any kind of server who says "I'm going to take this amount of tip" I recommend saying "No you aren't" and demanding the correct change. Tell them you get to decide what the tip is, unless they can show you the rules that say a tip is included in the price. A ticket seller should get zero tip, and anyone who tries to take a tip on their own initiative should get zero tip. Tips are a reward for good service, and trying to take money from you is not good service.

While five years later is too late to do anything, had you realised a few days later you might have considered contacting the place you visited and telling them about your experience. They will also probably be unhappy with their cashier, and you might get your money back.