Many of purchasing employees take a commission from the suppliers to submit their quotations with management, is that right or wrong?

I agree is unethical, they supply chain team shall be focused on finding the best suppliers with the best macth between price and delivery lead time, and not seek for advantages in order to introduce suppliers, as they are mainly performing theyr role.

My current and previous companies have policies to eliminate such practices.

Why can't say's that sometimes there's injustice from the company to the employee in terms of salary or benefits, which is forced them to do such an act?

This is not an excuse to betraying your company and conscience, but injustice is a strong reason and motivation for some people to do such things in compensation for the injustice of the work.

And the companies must give what they deserve and make the working environment suitable for all their potential.

If it does, and he does so, it may take all possible penalties on the employee after the conviction is found.

Indeed, it is wrong and should be taken as serious steps to avoid such malpractices.

It is unethical,

as their main role they are payed for is to get the best quotations for their employers. not in favor of any others