That's an interesting observation, a lot of us old folks (well, those who have children going through the education system) may remember going to school and having nothing but christian based assemblies with occasional "Asian" one thrown into the mix.
I think that's a decision parents have to take individually, if they feel that a Sikh ethos school should only mean they teach Sikhi then they don't fully understand how the education system works, i think what they are looking for is a Sikh religious school. My kids go to a Sikh ethos school in the midlands (not Nishkam), they have and take part in Sikh assemblies and ones from other religions, the make up of that school is majority Sikh, I've never heard of a parent complaining about it.
If by learning about another religion or taking part in an assembly somehow makes your belief in your religion less then that's something you need to work on, as i mentioned above, i grew up having to take part in Christian assemblies, my religion stayed resolute.
It comes from the need to acommodate and be all inclusive.
Our people feel the need to deprecate our needs and prioritise others over ourselves.
There are elements of gora society that does this and we follow suit.
It's not hate, people just disagree with their fanaticism for interfaith. In the new Niskham West London school they make sure that the kids who are a majority Sikh have to take part a Hindu, Muslim and Christian assembly during the week. There is probably just a handful of Muslim, Hindu and Christian kids in the scholl and yet the make the majority Sikh kids take part in these assemblies. Khalsa Primary School in Norwood Green does not do this and the have a daily divan in the Gurdwara Sahib.