What's this ? Mob justice for "blasphemy" against nationalism in a "civilized" secular state? Nationalism & secularism are indeed the religion of non-muslims in our time. Nationalist symbols r sacrosanct and not showing "respect" to these symbols is a social taboo. Freedom of religion , individual rights and free speech goes out the window in this case.
As for the muslim family , i don't know whether they did it due to religious or personal reasons , but they indeed did the right thing. Islam doesn't allow either singing or standing up for national anthem and muslims should not do that. Not to mention Islam prohibits music and nationalism. But more importantly what were they doing in a place of corruption like a movie theater?
http://www.islamweb.net/emainpage/index.php?page=showfatwa&Option=FatwaId&Id=83792
Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
As you have mentioned, it is Haram to stand for the national anthem. A prohibition of such an action depends on:
1) This action constitutes a form of glorification and sanctification, which is like worship.
2) This action does not only establish the principles of allegiance and loyalty on nationalism and patriotism, but also it is at the expense of allegiance to Allah, Islam and Muslims.
3) The national anthem is accompanied with music.
4) Such an action constitutes an imitation of Jews and Christians.
Muslims have to be distinguished among others nations and have to avoid the others' special characteristics.
Allah knows best.
http://www.islamweb.net/emainpage/index.php?page=showfatwa&Option=FatwaId&Id=85728
Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
al-Bukhari reported in 'al-Adab al-Mufrad' and al-Tirmizi in his Sunan from Anas(RAA) that he said: "They (the companions) did not love anybody in this life more than the Prophet (Sallallahu Alaihi wa Sallam) and they did not use to stand up when they saw him because they knew how much he disliked that" .
No doubt that standing up is an act of glory which originally should be rendered only to Allah. This is why it is one of the most gracious acts of prayer. It is narrated in a sound Hadith that the Prophet (Sallallahu Alaihi wa Sallam) said: "The best in the prayer is the long Qunut", i.e. standing up" . [ Ahmad andMuslim ].
So, since the Prophet (Sallallahu Alaihi wa Sallam) who is the best of all human beings and the one that Allah appreciates most, disliked this custom then it is unIslamic and it is a bad innovation which was introduced into the Muslim society by disbelievers.
The Prophet (Sallallahu Alaihi wa Sallam) said in a sound Hadith reported by al-Bukhari and Muslim : "You will follow the ways of those nations who were before you, span by span and cubit by cubit (i.e., inch by inch) so much so that even if they entered a hole of a lizard, you would follow them." We said, "O Allah's Apostle! (Do you mean) the Jews and the Christians?" He said, "Whom else?" .
So, it is unlawful to stand up for such a thing and Muslims should make sure that anything they do conforms with these rules. Blind imitation of disbelievers is rejected. It is reported from Abu Dawood that the Prophet (Sallallahu Alaihi wa Sallam) said: "Whoever imitates some people, he is one of them" . So, the Muslim should not commit this Haram unless there is dire necessity like fearing to be jailed or beaten. In this case, he may make believe that he is standing for the anthem but keeps on remembering Allah and should feel deep sorrow for doing such a thing. Indeed standing for these flags and anthems is a glorification of them. And if the person intends by such act any glorification of these things similar to glorifying Allah, then he becomes a Kafir, i.e. moves away from Islamic religion.
Allah knows best.