Why did FIDE allow some random guy to open for white in Chess Candidates tournament 2018? As seen in Aronian vs Kramnik on day 3,  the opening was e4 which Levon Aronian rarely plays. It later turned into the Berlin variation.

The article from FIDE says 


  Francesco Tristano, a classical and experimental pianist from Luxembourg, made the ceremonial first move for the round. By chance, it was in the game between Aronian and Kramnik, and he started with 1 e4, a move that Aronian almost never plays. 


Why did FIDE allow Francesco Tristano to open for white in Chess Candidates tournament 2018? What if the person chooses a "relatively" bad opening like f3 or h3?



source: https://www.youtube.com/watch?v=f86OTmsnI1c
It is very common in top tournament that a local celebrity or other people who like attention such as politicains, people from FIDE, etc, make the ceremonial first move. This is purely for the photographers, and of course it is perfectly fine to take back this move and make your own move.

In the game you mention however Aronian chose to stick with 1. e4 which is of course also an option. I don't think that Aronian was in any way influenced by the choice of Francesco Tristano, i.e. he would have played 1. e4 anyway.