It should not be. In Islam, a duty has no value to God unless it is done willingly, and the Quran says there is no compulsion in religion. Forcing hijab on a woman, as some families or governments do, is a cultural or political practice, not an Islamic ideal.
The fuller answer
Millions of women around the world wear it by choice, and also many Muslim women do not. In some countries, it is law and in others, such as in parts of Europe, it is banned in certain places. Muslims who value their faith criticise both: the right of a woman to choose what she wears is a principle, whichever way the pressure goes.
Pressure and abuse do exist in some families and societies, and Muslim women's groups speak out against them. The Prophet ﷺ taught that a person's actions are by their intentions, and that the one who fulfils a duty with love is rewarded more than the one who does it out of compulsion.
There shall be no compulsion in [acceptance of] the religion. The right course has become clear from the wrong. So whoever disbelieves in Taghut and believes in Allah has grasped the most trustworthy handhold with no break in it. And Allah is Hearing and Knowing.
And say, "The truth is from your Lord, so whoever wills - let him believe; and whoever wills - let him disbelieve." Indeed, We have prepared for the wrongdoers a fire whose walls will surround them. And if they call for relief, they will be relieved with water like murky oil, which scalds [their] faces. Wretched is the drink, and evil is the resting place.
This answer follows the mainstream Sunni understanding and is general learning, not a fatwa. Where Muslims differ, the answer says so. Have a question of your own? Write to us.