Definition: Given a travel related situation, and given a specific question about the situation, summarize the question into a title
Input: I am a Spanish citizen, and next summer I will go on a cruise trip from Spain to France (only stopping in those countries). The organising company seems to be oriented to a USA public so they require the passport for the reservation/registration of the travel as well as the embarkation.

I asked them if I really needed the passport, given that my National ID lets me travel through all EU countries, and they replied that I do need to show my passport when I embark the ship.

Why do they require me to bring the passport?

UPDATE: after contacting the company a second time, they answered that the cruise line requires that I have my passport with me although this company is aware of my IDs grants

Output:
Why do they require my passport to travel within the EU for a cruise ship?