Ruiz (33-1, 22 KOs), 29, of Imperial, California, the first fighter of Mexican descent to win a heavyweight world title, and Joshua are due to meet in a much-anticipated rematch.
The bout, dubbed "Clash on the Dunes", will see Joshua bid to win back the IBF, WBA and WBO titles which Ruiz Jr took from him in a stunning upset at Madison Square Garden in June, and will take place on December 7.
But Matchroom promoter Hearn defended the decision following rumours of Cardiff as a potential venue, citing other high-profile sports to have been played in Saudi Arabia while saying it could be a boost to boxing in general to broaden its audience.
"We have to realise there is another world out there outside Cardiff and Madison Square Garden", he said.
"For us, we really wanted to go somewhere who believed in the sport of boxing, that had a vision. It was a fantastic event, not just for the logistical setup but because of the vibrancy of the crowd, the interest in the sport of boxing".
Parker's promoter Eddie Hearn, who also promotes Joshua, has been forced to defend staging the Joshua versus Ruiz Jr rematch in Saudi Arabia. "You could be seeing a big change in the dynamics of the sport", Hearn told a London press conference on Monday. We already knew that Saudi Arabia were for real, we already knew they was investing in the sport of boxing and that was very important for us. Men and women are welcome.
Joshua is expected to earn well over £50million, his biggest payday to date, in spite of being the challenger, with his purses from Saudi Arabia, Sky Box Office in the United Kingdom and DAZN, the fighter's digital broadcast partners in the US. "If you buy a ticket you get an automatic visa, and it's six hours away".
"We understand some criticism but we also understand the people that have been there before us - organisations that are considerably bigger than us", he said.
"Look at the events that have been there before us - Formula 1 with the Formula E, of course; the European Golf Tour; the Italian Super Cup; World Boxing Super Series; the WWE - everyone has enjoyed these events from a fan perspective and a logistical experience as well".
"This is such a huge occasion for boxing, an iconic moment", said Hearn.
Hearn and Khalil said the fight will take place in a 12,000- to 16,000-seat open-air stadium that will be constructed just to host the bout in a city on the outskirts of the Saudi Arabian capital city of Riyadh.
"As with other sporting stars going to Saudi Arabia, we'd call on Joshua to inform himself of the human rights situation and be prepared to speak out", he added.
Joshua, who lost the International Boxing Federation, World Boxing Association and World Boxing Organisation belts to Ruiz in NY in June, will undertake a three-day press tour with the American early next month, taking in Riyadh, London and NY.