Look at it this way:
A.) The PLO takes a plane hostage and makes demands of the Israeli government.
B.) Cowboys take a Federal building hostage and make demands of the US government.
The cowboys to their credit did not take any people hostage, but the idea is still the same: if you are going to use arms as a way to change the law then you are basically privatizing war, and that can easily be defined as terrorism. In both cases they may have legitimate grievance with the government, but that doesn’t change the nature of the act. This is a far cry from dedicated non-violent protest, where the protesters are willing to be beaten and thrown in jail to make their point; Gandhi and MLK never threatened violence.