If you believe that telling people music is haram will prevent violent society of course I will laugh at you, but it is completely within the scope of secular society for you to say that.
What is not within the scope of secular society is enshrining within law compulsion on people to follow your religion.
The ideal of secularism you talk about (your religious worldview having no influence over law) is impossible. It doesn't really exist and never has.
Because all law is derived from morality (if it claims to be a just law it claims to be based on moral truth).
And you cannot separate an individual's sense of what is moral (right or wrong) from what they believe about the world (ie. Religion, or lack thereof).
So your demand and expectation that someone should divorce their sense of religious morality from what they advocate is moral for society is unreasonable - and logically impossible if they really believe in what their religion teaches is moral.
To even make law from an anti-religious worldview is functionally no different than making law from a religious worldview. You're still "imposing" your unproveable atheistic worldview on others because you think it's true.
So I have to wonder why such a foundational principle of justice (ie secularism)
You will find no historical basis for your belief that secularism is required for justice, or that justice comes out of secularism.
English common law isn't based on it nor is US constitutional law.
The entire idea of secularism as the basis for a society doesn't appear outside of bloodbaths like the French revolution and various communist revolutions.
The idea of secularism is found in the soviet constitution as one of lenin's goals and the principles of marxism.
But their idea of secularism is not what you've been told secularism is.
Their idea of "separation of church from state" was that religion would be effectively removed from society by not being allowed to have influence over it. Ie. "You can have your private religious service, as long as you stay in the closet and don't try to change society based on your religion".
They didn't want competing moral views.
And that's what you unknowingly advocate for when you talk about how you think someone's religion shouldn't be allowed to influence what policies they advocate for on behalf of society.
At no point in english common law history prior to communism, or US constitutional history, was that ever the operating belief about how society should be ideally structured. And you will find nothing to support your belief that it was the foundation of those justice systems.
appears to be redefined to suit the self serving purposes of the religious whiner in the clip?
I'm not sure what you're trying to argue, but the premise underlying your conclusion is not true, as I pointed out above.
What he's pointing out is logically true. It makes no sense not to advocate for islamic morality if you believe in islamic morality. Just like it makes no sense not to advocate for atheistic based morality if you believe in atheism (like how the nazis claimed their eugenics program was morally right based on the atheistic idea of evolution and natural selection).
You might think islamic morality is destructive to your anglo-saxon western society that was built upon a protestant christian foundation (and I might agree with you there) - but if that's the case then you should rethink your position on letting people from all over the world migrate into your country who don't share your worldview, morals, and values. Either that or you're going to have to force them to give up their existing worldviews and be willing to adopt those your nation currently holds to. Otherwise you're going to have to accept them attempt to change what your society values, or overwhelming you in numbers eventually so you're forced to accept what they value.
If you're expecting them to let go of that and integrate into your society then you will have a hard time doing that honestly without also advocating they abandon islam.
Or, at the very least, you must advocate they adopt a toothless version of islam that doesn't actually follow or believe in what their ancestors did.
Because you can't logically expect them to not advocate for what they think is moral based on islam if they truly believe islam is true.
The left tries to have it both ways but they can't. You can't advocate for unchecked muslim migration into your country but also not demand those muslims do anything to assimilate into your culture by modifying what they belief (because that would be "offensive" to suggest they need to change) and then act surprised when they want to change what your country's laws are based on the values and worldview they imported.
You've got two options:
1. Keep your society's values and laws the same by preventing immigration from those who don't hold views compatible with your society. Or require that those who do immigrate be willing to give up their conflicting views in order to embrace what your society believes in.
2. Let them come in and let them retain their existing views. Which risks changing your country's values and laws over time if they are a minority. And if they become a majority then it guarantees your country's values and laws will change.
You can have one or the other, but you can't have both at the same time.