I think it is a question of acoustics. I regularly sit in WSU and can confirm that they do join in when the NS gets its act together. If you are in the NS you probably can't hear. I sat in the ESL for the West Ham game and literally no-one around me bothered. But it's the same at most grounds...
I reckon you are absolutely correct. When I've been in the NS (I'm usually WSU) and looked around I could never figure out why there are so many people who look like the ES would suit them more. But you're right there was no ESU when we moved in and season tickets were cheapest in the NS...
If you sit in the home "end" it's your job to crank the singing-up and WSU and SW corner always join in. When I occasionally sit in the NS I notice that there is a large number of people in the middle zone, often older types and lots of women, who don't join in the singing and so the police...