He took the bar to the Manhattan Supreme Court, claiming the incident "offended his sense of being American".
On Wednesday, when the bar's lawyer, Elizabeth Conway, pointed out that only religious beliefs - not political ones - are protected under state and city discrimination laws, adding that supporting Donald Trump is not a religion, Piatek disagreed.
"The purpose of the hat is that he wore it because he was visiting the 9/11 Memorial," his attorney Paul Liggieri said.
"He was paying spiritual tribute to the victims of 9/11. The Make American Great Again hat was part of his spiritual belief," the lawyer added. "Rather than remove his hat, instead he held true to his spiritual belief and was forced from the bar."
The judge asked further questions about Piatek's "spiritual beliefs" relating to Trump.
"How many members are in this spiritual programme that your client is engaged in?" the judge reportedly asked.
"Your honour, we don't allege the amount of individuals," the lawyer replied.
"So, it's a creed of one?" the judge asked.
"Yes, your honour."
The judge ruled the incident was nothing more than a "petty" slight and determined that it was "not outrageous" for the bar to refuse service to the man for being a Trump supporter.