Yes, Uber does have the right to remove you from their service, for mostly any reason -- probably even because they think you're a "narc". But Greyball went way beyond that, and actively misled "suspicious" (probably sting agent) users into thinking cars would come, when they never did, wasting their time. That crosses over into interfering with an investigation -- the combination of "wasting someone's time" and "because that someone is an investigator".
Wasting the time of an abusive user? Probably okay. Kicking off law enforcement officers? Probably okay. Deliberately wasting the time of investigators? Probably crosses the line.
Compare two scenarios when an undercover officer is investigating a biker bar for dealing meth:
Scenario A: The officer seems somehow off to the bartender. Bartender glances around at his buddies, who shake their heads. "We don't want your kind in in here, get out."
Scenario B: Officer is identified as a cop. Bartender invites him to sit at a table far in the back with no view of anything. Any time the officer gets up, he's told to leave or sit back down. They have members stand within earshot of him and pretend to plan out fake meth deals at places where nothing will actually happen.
In A, the bar is on firm legal ground. In B, they're not, and that's basically what the Greyball program is.
Note: insights drawn from the earlier discussion about this https://news.ycombinator.com/item?id=13785564