At some point, while I agree it is shady and not an ideal practice, it kind of seems like it's illegal because business management doesn't like it or get a piece of it, not because it represents material harm.
The reason it is illegal is fiduciary responsibilty. What it boils down to is this: The people who own the company have hired this person to make business decisions which will maximize the profit (or maximize whatever specific metric was defined as his responsibility). When someone with a fiduciary responsibility takes a bribe, they are basically conspiring with the bribing entity to steal a money from the shareholders/business owner and taking a cut of that stolen money in the form of the bribe.
ie: You give me $100 and tell me to go get the best value widget on the market. I determine that widgets should cost $50. Company A offers the widget for $50 and Company B offers the widget for $80.
Company B offers a bribe of $20 dollars if I choose their $80 widget. This provides for $10 pure profit for company B over the base widget value, and $20 for me by accepting.
However, that $30 difference was $30 that was effectively stolen from you when the decision was made to not select the appropriately priced $50 widget.
The value of the bribe, plus the cost difference between comparable products is pure theft from the people whose money the bribe receiver was entrusted to manage.
That's why it is illegal, and unethical.