The servicemen are not accused of aiding the 21-year old Saudi Air Force lieutenant.
But US Attorney General William Barr said the cadets were found to have had jihadist material and indecent images of children in their possession.
Three sailors were killed and eight wounded in the 6 December attack.
Training for Saudi servicemen was put on hold in the US after the attack.
Mr Barr told a news conference on Monday that the shooting at Naval Air Station Pensacola had been an “act of terrorism”.
He said he had asked Apple to unlock two iPhones that belonged to the gunman, who was killed by police in the attack. The gunman fired a bullet into one phone in an effort to destroy it, Mr Barr said, but FBI investigators were able to restore the device.
“We have asked Apple for their help in unlocking the shooter’s iPhones,” Mr Barr said.” So far Apple has not given us any substantive assistance.”
Apple had given the FBI iCloud data from the attacker’s online account, the New York Times reported, but refused to unlock the phone, saying it would undermine their own encryption software.
The tech firm has clashed previously with the FBI over requests to unlock iPhones belonging to terror suspects. A similar 2016 clash was resolved when the FBI found a way to unlock a phone belonging to a mass shooter in California without help from Apple.
Mr Barr said that initial reports that other Saudi cadets had filmed the attack as it unfolded were inaccurate. The gunman had arrived at the scene of the shooting alone, he said.
The attorney general said 17 of the expelled cadets were found to have possessed online terrorist material. Fifteen, including some of the 17 who possessed online terrorist material, had indecent images of children, he added.
“While one of individuals had a significant number of images, all the rest had one of two images, in most cases posted in a chat room by some other person or received over social media,” said Mr Barr.
He said the 21 cadets were being disenrolled and returned home on Monday. The Saudi cadets, he said, had fully co-operated with FBI investigation.
Mr Barr also said the Kingdom of Saudi Arabia had given “complete and total” support to the inquiry. Saudi officials had determined the cadets’ conduct was “unbecoming an officer in the Saudi Royal Air Force and Royal Navy”, said the attorney general.
He added that the expelled cadets had not been charged with any crime in the US, but might face prosecution back home. There are more than 850 Saudi military cadets conducting training in the US.
Investigators say the attacker, Second Lt Mohammed Alshamrani, had shown videos of violence to his colleagues at a dinner party before the attack. The 9mm handgun he used was purchased lawfully.
Asked about the planned expulsions on Sunday, White House National Security Advisor Robert O’Brien told Fox News the Pentagon had decided to expel the Saudi cadets.
“Obviously Pensacola showed that there had been errors in the way that we vetted,” said Mr O’Brien.
“And I think out of an abundance of caution Secretary [of Defence Mark] Esper’s taking these actions to protect our service men and women.”
The Pensacola base has long offered aviation training to foreign military forces. Saudi pilots started training there in 1995, alongside other personnel from Italy, Singapore and Germany.
After last month’s attack, the base’s commanding officer said that about 200 international students were enrolled in programmes there. According to its website, the base employs more than 16,000 military and 7,400 civilian personnel.