Championship leader Norris had finished second in the race behind Red Bull's title-chasing Max Verstappen with Piastri in fourth.
However, in the post-race inspection, the skid blocks on both cars were found to be less than the minimum depth, resulting in both cars being disqualified.
"We apologise to Lando and Oscar for the loss of points today, at a critical time in their championship campaigns after two strong performances from them all weekend," said Stella in a McLaren statement.
"During the race, both cars experienced unexpected, high levels of porpoising not seen in the practice sessions, which led to excessive contact with the ground
"We are investigating the reasons for this behaviour of the car, including the effect of accidental damage sustained by both cars, which we found after the race, and that led to an increase in movement of the floor.
"As the FIA noted, the breach was unintentional."
Norris began on pole but lost the lead to Verstappen on the first turn. He then had to nurse his McLaren through the final few laps allowing the Dutchman to finish more than 20 seconds clear.
But second place appeared to give him a valuable 30-point lead over Piastri going into the last two race weekends of the season.
"It's frustrating to lose so many points," said Norris who now holds a 24-point lead over both Piastri and Verstappen.
"As a team, we're always pushing to find as much performance as we can, and we clearly didn't get that balance right today.
"Nothing I can do will change that now, instead full focus switches to Qatar, where we'll aim to go out and deliver the best possible performance in every session."
Piastri said it had been "disappointing" to leave Vegas empty-handed.
"We're always looking at where we can gain performance, and we didn’t get it right this time," said the Australian.
"We now need to reset, refocus and push to get the best points possible in the final two rounds."
With a maximum of 58 points available in the final two grands prix, Norris can win the title in Qatar if he earns two more points than both Verstappen and Piastri over the course of the weekend, which includes a sprint.