This is important because banning a piece of art based on speculation or threats by groups with various ulterior motives is unfair and goes against the very fabric of a democratic society. If the Islamist groups and the film maker have reached a consensus, there is no reason why a state should interfere in this matter.

Article 19 of the Universal Declaration of Human Rights states that Freedom of Speech is the right of every human being. Since India is party to this Declaration, it is the duty of the President of India to uphold this. It is not just Kamal Hassan's right to expression that is at stake here. It is also the rights of all Indian citizens who are unable to access the film, that is being denied.

For Reference:
What does The Article 19 of the Universal Declaration of Human Rights state?

1. Freedom of expression is the right to speak

It is the right to voice political, cultural, social and economic opinions
It is the right to dissent
It makes electoral democracy meaningful and builds public trust in administration.

2. Freedom of expression is freedom of the press

It is the right of a free and independent media to report without fear, interference, persecution or discrimination
It is the right to provide knowledge, give voice to the marginalised and to highlight corruption
It creates an environment where people feel safe to question government action and to hold power accountable.

3. Freedom of expression is the right to know

It is the right to access all media, internet, art, academic writings, and information held by government
It is the right to use when demanding rights to health, to a clean environment, to truth and to justice
It holds governments accountable for their promises, obligations and actions, preventing corruption which thrives on secrecy.