Attendance was optional, but contract workers who chose not to stand in the crowd would not qualify for time-and-a-half pay when they arrived at work on Friday, the Pittsburgh Post-Gazette reported.
Workers at the unfinished Pennsylvania Petrochemicals Complex had to arrive at 7am, scan their ID cards and stand for hours until the US president’s speech began.
“NO SCAN, NO PAY,” a supervisor for one of the contractors wrote to workers.
The contractor’s memo also banned yelling, protesting or “anything viewed as resistance” at Mr Trump’s speech, according to the newspaper.
“An underlying theme of the event is to promote good will from the unions,” the document said. “Your building trades leaders and jobs stewards have agreed to this.”
Several companies with thousands of unionised workers have contracts with Shell, one the world’s largest oil and gas companies.
The Washington Post was unable to immediately reach Shell or the plant’s unions for comment on Saturday.
Mr Trump has a long history of falsely claiming that liberal demonstrators have been paid to protest. When people angrily flooded the streets of some cities after he won the presidency, he accused them of being “professional protesters” who had been “incited by the media”.
When women protested Brett Kavanaugh’s nomination to the Supreme Court, he said they were “paid professionals”.
And when protests bubbled up at airports in 2017 in response to Mr Trump’s ban on immigrants from seven Muslim-majority countries, he alleged that the demonstrators were “professional anarchists, thugs and paid protesters”.
The president’s speech on Tuesday felt at times like a campaign rally, The Washington Post reported.
Between remarks about US energy production, Mr Trump urged the workers to support his re-election and complained about his perceived enemies: the media, the Democrats running for president and the Academy Awards.
About 5,000 workers attended the speech, according to Newsweek.
Shell spokesman Ray Fisher told the Post-Gazette that workers at the plant have a 56-hour workweek, which includes 16 hours of overtime pay – so workers who showed up on Tuesday were paid for the week at a higher rate.
Another Shell spokesman, Curtis Smith, said workers who chose to skip the rally received “paid time off”, which does not count as hours worked and therefore does not trigger overtime pay.
Mr Trump’s speech was treated as a training that differed from other training sessions only in that it included “a guest speaker who happened to be the president,” Mr Smith said.
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“We do these several times a year with various speakers,” he told Newsweek. “The morning session (7-10am) included safety training and other work-related activities.”
Ken Broadbent, business manager for the union Steamfitters Local 449, said his workers respect Mr Trump for his title, regardless of whether they liked or disliked him.
Anyone who did not want to go to work on the day of Mr Trump’s speech could skip it, he said.
“This is just what Shell wanted to do and we went along with it,” Mr Broadbent said.
The Washington Post