It would not if there wasn't proof that there was actual planning. That said, you would most likely be monitored by local and state law enforcement.
A similar case happened in Central Illinois a couple years ago, where threats were posted but arrests were not made until the threats moved to actual action.
Not in the US's case. A direct link would be needed.
American jurisprudence on speech leans towards Free Speech Absolutism [0] due to jurisprudence from the 1970s-2000s, and the test for "clear and imminent danger" is extremely high.
Even though the US and the rest of the Anglophone speak English, America jurisprudence is extremely distinct from the rest of the Anglophone (and vice versa), and IMO it doesn't make sense to compare one with the other due to these significant differences.
For example, the UK dealt with the Troubles into the late 1990s, and the US never had a similar insurgency since the 1950s in Puerto Rico, so there is a hardening in NatSec laws in the UK compared to the US.
This is why the US often leverages allied states to help with this kind of monitoring to sidestep some of the legal implications domestically.
That said, I agree with your point to a certain extent, the issue is the US and other Anglophone countries have a different relation with speech and civil liberties. It doesn't make sense to compare the US with the UK or EU and vice versa.
How direct is direct enough? Connolley posted messages inciting racial violence, racial violence ensued.
Was Connolley a major instigator of these riots? No.
The judge's sentencing remarks are below, the key part being:
>6. When you published those words you were well aware of how volatile the situation was. As everyone is aware, that volatility led to serious disorder in a number of areas of the country where mindless violence was used to cause injury and damage to wholly innocent members of the public and to their properties.
Connolley's message was posted at 8.30pm on the 29th July. One day before crowds attacked Mosques in Southport. Related disturbances continued until August 5th.
A similar case happened in Central Illinois a couple years ago, where threats were posted but arrests were not made until the threats moved to actual action.