The Australian's car was hit by Sauber's Nico Hulkenberg at the first corner and pitched onto Norris, who had been on the front row and was chasing Red Bull's eventual winner Max Verstappen.
The safety car was deployed, with debris across the track at Austin's Circuit of the Americas, and the collision became the day's main talking point.
"I think it is just a racing incident. Lando and I were a long way away from the apex and it’s impossible to see everything at that point," said Piastri.
"You have to trust your gut and your instinct, and that’s what I did."
Norris retired by the side of the track and Piastri, 22 points clear of his teammate at the top of the standings, entered the pits with a broken suspension.
"That was terrible. Neither of our drivers to blame there," McLaren boss Zak Brown told Sky Sports television while the race was still going on.
"That's some amateur-hour driving by some drivers up there at the front, whacked out two guys."
After looking at video replays, the American changed his tune however.
"I've reviewed it, I think I've changed my view. I can't really put that on Nico," he said.
Team principal Andrea Stella told reporters after qualifying for Sunday's main race that some very experienced drivers should have shown more prudence.
Norris refused to blame Piastri when reporters asked if he felt the Australian was over-ambitious in cutting back aggressively at the tight first corner with two cars on the inside.
"He got hit, no? He got hit, right? So why is it his fault?" he asked.
Later, the Briton said he would understand more after the weekend.
"I think things will be reviewed just to understand a few more things. But, apart from that, it's nothing I can do. I just have to crack on and do my own thing."
The incident was the second successive weekend of contact between the McLaren pair, with Norris forcing Piastri wide in Singapore.
That incident triggered talk of consequences and repercussions, with Norris blamed.