Freedom of speech does not imply freedom from responsibility for the speech spoken. Some speech, depending on its content, is just as much speech as it is an act that leads to actions by more people that have tangible consequences - such as military orders, fires in crowded theatres or declarations of war. Admittedly, the line is very thin and hard to see when you are close. But, while this speech is free and should remain so, the act, which in this case qualifies as Incitement to Genocide under international law[see below], can and should be treated accordingly. That is the subtle but relevant difference between "I hate Americans" and "Let's crash a plane at them".
Article 3
The following acts shall be punishable:
(a) Genocide;
(b) Conspiracy to commit genocide;
(c) Direct and public incitement to commit genocide;
(d ) Attempt to commit genocide;
(e) Complicity in genocide.
and...
Article 2
In the present Convention, genocide means any of the following acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group, as such:
(a) Killing members of the group;
(b) Causing serious bodily or mental harm to members of the group;
(c) Deliberately inflicting on the group conditions of life calculated to bring about its physical destruction in whole or in part;
(d) Imposing measures intended to prevent births within the group;
(e) Forcibly transferring children of the group to another group.
Source: http://www.unhchr.ch/html/menu3/b/p_genoci.htm