Read this: Eritrea is a one-party state in which national legislative elections have been repeatedly postponed. According to Human Rights Watch, the government's human rights record is considered among the worst in the world. Most Western countries have accused the Eritrean authorities of arbitrary arrest and detentions, and of detaining an unknown number of people without charge for their political activism. However, the Eritrean government has continually dismissed the accusations as politically motivated. In June 2015, a 500-page United Nations Human Rights Council report accused Eritrea's government of extrajudicial executions, torture, indefinitely prolonged national service and forced labour, and indicated that sexual harassment, rape and sexual servitude by state officials are also widespread.

According to many western countries, why have Eritrean authorities detained an unknown number of people?
What is the answer? (If it cannot be answered, return "unanswerable")
for their political activism