The classic analogy is that a person shouldn't be allowed to shout "FIRE" and thereby cause panic in a crowded theater, when there is no such fire. The right of the one person to (falsely) speak is outweighed by the right of the many people present to their own safety.
Trump saying "I WON THIS ELECTION, BY A LOT!" [1] is clearly equivalent to shouting "FIRE". He has not won the election, he does not have more popular votes or more electoral college votes, and he has presented no proof or substantive argument to contradict this narrative.
That Twitter should put a warning label on statements from powerful people that are objectively false is really the least they can do. To let Trump spread lies without any comment before now has been wholly irresponsible. I wish that Twitter didn't provide a platform for misinformation in the first place.
[1] https://twitter.com/realDonaldTrump/status/13250998450450718...