"Probably because it's easier to contain evidence of a (commercial) brown envelope than a discount."
It's likely to be far more prosaic. Discounting hits your product margin and your top line billings. This makes Wall Street analysts (who depend heavily on these two numbers) very, very unhappy. Dressing up the equivalent discount as a one-off payment can be accounted for any number of ways that don't impact your share price.
Also let's say you form an agreement for 3 years of exclusivity and after 2 your customer decides to end the agreement. It's hard-to-impossible to take back a per-unit discount, while it's very straightforward to claw back a one off payment, or even not pay it at all.
Accountants make the world go round.
Biting the hand that feeds IT
