"Even if Google had, for the sake of argument, terminated the employees for their protest activities -- for protesting their choice of customers -- this would not violate the Act", Google's attorney Al Latham said in his opening statement at a labour board trial.

 National Labour Relations Board prosecutors have accused the Alphabet unit of violating federal law by illegally firing five employees for their activism.

Three of those workers' claims had originally been dismissed under President Donald Trump, because agency prosecutors concluded that their opposition to the company collaborating with immigration enforcement wasn't legally protected, according to their lawyer.

But that decision was reversed after President Joe Biden fired and replaced the labour board's general counsel.

Google has been roiled over the past four years by a wave of activism by employees challenging management over issues including treatment of sub-contracted staff, handling of sexual harassment, and a contract with the US Customs and Border Protection agency, which some of the fired workers accessed internal information about and circulated a petition against.

 Google has denied wrongdoing, saying in a Monday statement that it encourages "open discussion and debate" but terminated staff in response to violations of its data security policies.

"Google terminated these employees not because of their protest as such, but because in the pursuit of their protest, they accessed highly confidential information that they had no right to access", its attorney told the judge.

Federal labour law prohibits retaliating against employees for collective action related to their working conditions, but the exact scope of that protection has been debated for decades.

Biden's appointees have signalled they interpret the scope of what that covers much more broadly than their Trump-era predecessors.

Latham said he isn't aware of any case in the labour board's eight decades of existence in which it has held "an employer's choice of customer" to be an issue workers have a right to protest.

"What we have here is a protest that does not seek to improve employees' terms and conditions of employment," but rather "a purely political protest that sought to use Google's government contracts, or potential government contracts, as leverage", he said.