It may be within Twitters terms of service to ban people and censor people, and that may be fine for Twitter, but for a sworn officer to use Twitter to censor people - that is an abuse of power.
Of course, the files also show that Twitter personnel acted in bad faith and didn't follow their own ToS.
However if you think about it for a minute, if the FBI were this bold in written, subpoena-able communication, what do you think the offline conversations looked like?