Because the data was made available without restriction or use prevention by Facebook. The data they gained was technically freely available to anyone with the right tools - they just scraped thousands of open security Facebook users to get it. A lot of people have the wrong thinking on internet use - anything you put onto the internet is there permanently and can be accessed by others, depending on the level of security of the data. SnapChat, Instagram and others, regardless of what they promise about the volatility of your data, cannot prevent others from capturing it. Facebook is wide open, if you fail to set your security permissions properly, and that makes all the data it contains available to the right software. This company was not violating any laws against access to the data - it was the purpose they put it to over the years that broke fair use laws.
Cambridge Analytica ran voter suppression campaigns, whistleblower claims. The whistleblower whose disclosures about Cambridge Analytica shook the tech world over questions about users' data privacy told Congress on Wednesday that the company engaged in efforts to discourage or suppress voting from targeted sections of the American population.
Christopher Wylie, a former Cambridge Analytica employee, said that Trump's former adviser Steve Bannon used the company in an effort to suppress the black vote in 2016.
"Mr. Bannon sees cultural warfare as the means to create enduring change in American politics. It was for this reason Mr. Bannon engaged SCL (Cambridge Analytica's parent company), a foreign military contractor, to build an arsenal of informational weapons he could deploy on the American population," Wylie claimed. Ref. CNN.