According to them, as part of their defense in a defamation lawsuit filed against them in Canada.
https://thepostmillennial.com/bc-court-allows-twitter-to-be-sued-for-defamationTwitter has stated that they intend to defend themselves by arguing that they are not the publisher of the tweets, further claiming in court documents that they do "not mediate or review the tweets."
I'm sure this will be good news for people whose accounts have been locked ...