1- In general, showing loss of hope, despair, or objection to Allah’s will in times of tragedy is discouraged. Culturally, if one would hit their thigh, it indicated lack of patience and objection. This can change over time or it can be different in other societies. When we hit our chest or thigh for Imam Hussain (a) we are not objecting. We are showing our support for his cause. We are saying no to oppression. In fact, if anything, we are finding hope and getting closer to Allah.
2- Imam Hussain is an exception. Sahih hadiths tell us جزع which is despair or severe grief is makrouh except on Imam Hussain. Allah has made this exception for Imam Hussain.