"I was trying to let Helio go and he didn't go to the outside. That's why I moved over to the inside but he didn't go."
Power was fuming after being told of the penalty, with team boss Tim Cindric telling him to "get your head on straight" over the radio.
He was later seen speaking to IndyCar's president of competition Derrick Walker.
It's the fifth time this season Power has found himself on the wrong end of race control officials, including a pit-lane speed violation at the Indy 500 and in Texas as well as a contact penalty in Detroit.
Castroneves insisted there were no hard feelings between the pair after the race.
"That's one thing about our team, Team Penske, there are no team orders," he said.
"We race hard and we push for it.
"Unfortunately, I'm not the one to make the calls. But I think we were really pushing hard and obviously fighting for the championship."
Rookie Carlos Munoz finished the 200-lap race in third, ahead of Power's compatriot Ryan Briscoe in fourth and New Zealand's Scott Dixon.
The series next heads to Iowa for the Indy 300.
-AAP