The justification they used for banning him was based upon mind-reading and weasel words.
"the President’s statements can be mobilized by different audiences"
So can any number of statements from Democrat politicians and media allies. In fact, in view of the antifa/blm riots, arsons, and looting, it is reasonable to conclude that they were.
"is being received by a number of his supporters"
Who? Where? When? How do they know? And so what? One can't be responsible if a supporter or fan misconstrues some innocuous statement like "I'm not going to this event".
"The second Tweet may also serve as encouragement"
Again, same thing. We can find numerous statements that are outright calls to action from Democrat politicians ("fight them in the streets", "kick them", "create a crowd", "tell them they're not welcome anywhere", etc) as well as tacit support by media allies calling riots peaceful, and hey, if they turn violent, sometimes that's just what it takes.
The simple fact is Twitter, and big tech, supports Democrats and wants them to win. This is seen not only by their actions, but leaked communications from Twitter, Google, Facebook, and others. Remember the Googlers crying in the hallways, or given the equivalent of grief counseling after the last election? Not to mention their political donations are public record, not only corporate and PACs but employees.
The simple fact is there is a standard one side is held to, on which the other side is allowed massive leeway and benefit of doubt, on the simple basis of politics.
And the Parler move WAS cowardly. One can easily find many threats of riot, violence, insurrection, assassination, and mass purging of undesirables on Twitter et al, yet because the decision makers at those companies agree, they're allowed to stand.
If Trump did anything positive in his four years in office, it is exposing what tech and media elites and decision makers really think of those they purport to serve.