I'm more interested in why it took them this long, while they're perfectly fine suspending accounts that are politically opposing to their worldview. How are political dissenters more dangerous than actual terrorists?
As a tactic, terrorism describes the infliction of civilian casualties meant to create a disproportionate climate of fear. By this definition, drones used in warfare are a form of terrorism.
In today's world we are not fighting wars in the traditional sense of defending ourselves against aggression. Since there is no obvious threat, the people will not support a pragmatic war of aggression meant to help install US hegemony in oil rich (or otherwise geopolitically important) regions of the world.
So our planners are left with the need to create a moralistic crusade and to marginalize and dehumanize the adversary so that Americans will not care when we find out that many of them (and their civilians) were slaughtered or forced to live in poverty/squalor.
It starts with the framing of the adversary as helpless against an illegitimate regime. The people are framed as victims of their own failure to end up with a better regime, and the moralistic idea of nation building and bringing in democracy is introduced.
Then, the regime is asked to give up power. When it refuses, as all regimes would, the US pretends it has a moral justification to use force against people and infrastructure. There is the heavy implication that a greater purpose exists, such as democracy or women's rights, or stopping the torture policies of the regime, etc.
But in fact no plan exists other than to remove the regime from power. Never before has there been a more hollow policy/intellectual movement than the idea of nation building.
Meanwhile, the US has typically funded various militant groups that oppose the regime, and now those groups are no longer needed. Some decide to seek power in the newly existing power vacuum. They will be branded as terrorists arbitrarily to suit the needs of the US PR campaign.
The word terrorism is used to characterize a person or a set of beliefs or an organization as unworthy of respect and unworthy of humane treatment.
Yet in the US we fail to recognize our drone strikes as the same tactic as suicide bombings, while we create untold pain and suffering abroad.
Both of our major political parties strongly support the endless war and the endless killing and destruction of the political adversaries abroad who dare to try to control their own destinies and the destinies of their region.
Twitter clearly supports this too, and feels obligated to help eradicate the first amendment as much as possible in its own small way, even if it can't actually revoke the amendment.