Why is the Supreme Court's decision regarding habeas corpus during the Emergency regarded as controversial?

It allowed the government to curtail citizens' rights.

Answer: It allowed the government to curtail citizens' rights.
The Supreme Court's decision in April 1976, which allowed the government to take away citizens' right to life and liberty during the Emergency, is regarded as controversial because it curtailed citizens' rights.

The government made extensive use of preventive detention. Under this provision, people are arrested and detained not because they have committed any offence, but on the apprehension that they may commit an offence. Using preventive detention acts, the government made large scale arrests during the Emergency. Arrested political workers could not challenge their arrest through habeas corpus petitions. Many cases were filed in the High Courts and the Supreme Court by and on behalf of arrested persons, but the government claimed that it was not even necessary to inform the arrested persons of the reasons and grounds of their arrest. Several High Courts gave judgments that even after the declaration of Emergency the courts could entertain a writ of habeas corpus filed by a person challenging his/her detention. In April 1976, the constitution bench of the Supreme Court over-ruled the High Courts and accepted the government’s plea. It meant that during Emergency the government could take away the citizen’s right to life and liberty. This judgment closed the doors of judiciary for the citizens and is regarded as one of the most controversial judgments of the Supreme Court.