It's true that most of it is on unidays for free. It's also true that NUS is a massive organisation that isn't represented in its whole by the cases you've linked. Its also true that NUS is largely ignored by the vast majority of students, doesn't affect their lives past the discounts, and isn't something they're majorly interested in. The ones boycotting are a particularly vocal subsection.
But the main point is, it's not any of their parents business. They can decide themselves who they want to support, and which issues they want to give their limited political attention to.