Answer 1
Because the policy of USA is hijacked by Zionists and Zionists hate Shia Islam too much and so being terrorist is only an excuse to destroy Shia Islam in the world. Hezbollah is not seeking power in Lebanon and is a good group and not aggressive but defensive.
Answer 2
The question as written makes a major assumption about the nature of Hezbollah, namely that it is a defender of the Lebanese people. While this is certainly a message that Hezbollah espouses, it is quite incorrect.
Hezbollah is a belligerent militant organization whose primary goal is to create a Shiite Theocracy in Lebanon. There are vast numbers of Lebanese who would be directly harmed or threatened should such an eventuality occur, namely Maronite Christians, Sunni Muslims, and other minorities, who would lose rights or station. Secondly, and more importantly, Hezbollah does not actually secure any benefits for the Lebanese people in terms of protecting them from foreign enemies. Hezbollah killed numerous Lebanese people in the Lebanese Civil War while fighting alongside the Syrian Army. They also fired the first shots in the 2006 Israel-Hezbollah War by kidnapping two Israeli soldiers. During that war, Hezbollah further increased civilian casualties by drawing Israeli soldiers into small villages, resulting in a high level of urban warfare and a high level of civilian casualties. Blame for this direction of the war also lies squarely at Hezbollah's feet. Finally, Hezbollah opposes the will of the Lebanese people. In 2009, when Hezbollah did not form part of the Lebanese Government (instead being part of the Opposition), they brought tanks into Central Beirut and demanded concessions. This violates the principles of democracy that the Lebanese people have agreed to follow and is directly opposed to their wishes.
Hezbollah endangers civilians, has a violent ideology, and discusses the murder of foreign civilians almost daily. That in combination with its thousands of soldiers and its use of violence to terrorize its enemies, has resulted in the USA and many European countries' quite reasonable naming of Hezbollah as a terrorist militant organization.
