In the United States the terrorist designation process is legally limited to foreign organizations and not allowed for domestic ones because there is a fear that allowing that could be weaponized against Americans or political foes.
Reuters OP: "It was not clear what legal weight Trump's proclamation carried... legal experts said such a step lacked a basis in law"
See also:
https://www.voanews.com/a/usa_why-domestic-terrorism-not-spe... ("Why Domestic Terrorism Is Not Specifically Designated a Crime in US")
> "While U.S. law makes it a crime to provide “material support” to a foreign terrorist organization, there is no comparable law that makes domestic terrorism a federal crime, even though individual acts committed by domestic terrorists may be illegal."
If you break laws and conduct terrorism then yeah, breaking the law is illegal. Because breaking the law is illegal. Does not change anything I wrote or make what I wrote 'technically correct'. The United States military now extrajudicially just executes people the President designates to die claiming they are 'drug terrorists'. There is no US mechanism for him to flag American organizations in this manner (currently). Probably again, the worst kind of correct thing for me to say.