So those who earn below the threshold cumulatively across the don't have to pay more?
Eg students who only work in the summer, people on maternity pay, people who lose jobs, or reduce during the tax year. Anyone whose income massively fluctuates. If it was tax then they'd be able to claim it back. With NI they'll just be more out of pocket as you can't claim back in the next pay period or from HMRC :(