Echoing remarks from its statement last week, Delta noted the decision was preliminary and said it was confident regulators "will conclude that no USA manufacturer is at risk" from Bombardier's plane.
Bombardier has been hit with another trade tariff, as the worldwide dispute that threatens thousands of jobs in Northern Ireland rumbles on.
Boeing alleges the unfair subsidy stems from when Northern Ireland's power-sharing administration and the UK Government pledged to invest almost £135m to help set up a C-Series manufacturing site in Belfast.
British Prime Minister Theresa May said she was "bitterly disappointed" by last week's decision, considering Bombardier employs more than 4,000 people in Northern Ireland.
Boeing complained to the American government about Bombardier earlier this year after the Montreal-based company secured a deal to sell CSeries planes to Delta airlines last year.
A final decision on any USA duty is expected next year.
An independent US trade body still has to rule on whether Boeing suffered any harm from Bombardier's tactics when the Canadian company won a big sale of 75 CSeries jets a year ago to Delta Air Lines Inc. and unsuccessfully bid to sell planes to United Continental Holdings Inc.
"This was an avoidable outcome within Bombardier's control".
Montreal-based Bombardier said in a statement that the Commerce Department's decision was an "egregious overreach" and "misapplication" of trade law that is created to "block the CSeries aircraft from entering the USA market". "The laws governing global trade are transparent and well known".
"We will continue to verify the accuracy of this decision, while do everything in our power to stand up for American companies and their workers". Bombardier has a large aerospace plant in Northern Ireland.
The wings for the jet are made in Northern Ireland and Bombardier has said the C-Series model affected by the row is "critical" to its operations there.
The punitive tax would significantly raise the price of the jet in the United States market, and threaten the future of the product.
To win its case before the ITC, Boeing must prove it was harmed by Bombardier's sales, despite not using one of its own jets to compete for the Delta order.
U.S. Commerce Secretary Wilbur Ross said the decision affirmed Trump's "America First" policy.
The firm also took umbrage with Bombardier receiving $1bn (£765m) from the Canadian provincial government in Quebec in 2015, when its fortunes appeared to be flagging.
The US trade commission is due to rule on the Department of Commerce's 220% tax proposal next year, but ISME said the European Union should not wait for the final decision.