Why is it that certain nationalities are allowed just to change license while others are tested?

Similarity/differences in driving regulations/common driving habits of the country of the origin (NOT DISCRIMINATION)

Why are certain nationalities can have visa-on-arrival while others are not.

Likelihood of the person to stay in that country illegally based on economical status of the his country of origin (NOT DISCRIMNATION)

Why is it that salary is BASED on nationality?

Currency rate exchange (NOT DISCRIMINATION)

Why is it that accommodation benefits are different based on nationalities (not all though)?

Finally yes that is discrimination