eight is a postpaid plan, however it requires pre payment, and functions exactly like a prepaid card. it is the exact same with Heya, simba, every other MVNO where if you don't pay first month in advance you will get your line terminated.
If you like a normal postpaid plan with pay per use and bill shocks, your options are limited to Singtel sim only, starhub sim only, and M1 sim only.
It is not hard to understand there are postpaid plans that require pre payment. it is the norm nowadays because of the cheap plans.
Personally port my postpaid plan out of Heya, and by your logic that is a "pre paid plan", but in fact actually it is postpaid
You can assume i am speculating, but take a big look at any of the telco FAQs, you will be surprised.