The Ferrari driver had finished the race around the Hard Rock Stadium in fourth position, trailing team-mate Charles Leclerc, but falls one place behind Red Bull's Sergio Perez in the revised classification after being hit with a five-second penalty.
Sainz had reached fourth place after a lengthy battle with McLaren driver Piastri, who had passed Leclerc to run second early on before a safety car intervention altered the strategic outlook of the race and helped Lando Norris score his maiden F1 victory.
Watch: F1 2024 Miami GP Review - Lando Seizes his Historic Maiden Win
With the duo battling over fourth, Sainz was left irate after Piastri went unpunished over what the Spaniard perceived to be an overly defensive move into Turn 11, when he was edged wide.
Following persistent radio messages questioning why no penalty was forthcoming, Sainz went back on the attack when informed that no further investigation would take place.
Closing in on Piastri with the DRS heading towards Turn 17, Sainz lunged to the inside before a rear-end slide resulted in him swiping across the McLaren's front wing. This forced Piastri into a pitstop for a new wing, costing him any chance of points.
After investigating the incident, the stewards determined Sainz was guilty of causing a collision and handed him a five-second penalty, as well as one penalty point.
The stewards' report read: "It was clear to us that Car 55 was to blame for the collision.
"In the overtake attempt, Car 55 braked late, missed the apex and in the process lost the rear, with the resulting collision. Although Car 81 was trying to turn in to counter the overtaking attempt, Car 81 gave sufficient room to Car 55.
"In the circumstances, we find Car 55 to be predominantly to blame for the collision."
The report did stress, however, that "mitigating factors" caused the incident, adding: "The fact that but for the slight loss of control of the rear by Car 55, the collision would likely not have happened and it would have been hard but good racing.
"The standard penalty for a collision is 10 seconds with 2 penalty points. In light of the mitigating circumstances, we therefore impose a 5-second time penalty with 1 penalty point."
