CPF is seen as a tax only because one is never able to realize
it as hard cash, unless one turns 85 or thereabouts. In countries
like India, where the rate of contribution is 12% each for both
employee and employer, people are allowed to close their CPF
account whenever they leave a job, and start a new one with a new
employer.
All the accrued money (contribution plus annual interest of
8.5%) is credited to your account within a few months. So, though
employees have to compulsorily contribute to CPF, they don't
complain as they see it as a nestegg, savings they can make good
use of when they quit a job, or retire.