It seems the practical difference is mainly in name only, although it's possible that immigration officers are given more leeway to restrict entry for the "visa on arrival" countries.
I'm not sure if that's right. Countries all the time have mandatory fees included in Airline tickets. Sometimes those fees are paid at the airport on arrival instead. Many other other Visa Free countries have a mandatory departure tax.
A lot of the "Visa Free" countries have fees (whether they call them visa fees, tourism fee, etc). I'm not sure that any correlation can be drawn between "Visa Free" and "$0".
It seems the practical difference is mainly in name only, although it's possible that immigration officers are given more leeway to restrict entry for the "visa on arrival" countries.