Yes, I see your point; slashing base salary is different than not paying a bonus or significantly reducing it. However, the net effect is still the same: employees take home less money, and their estimation of how much they can "expect" to get is thrown off. The change here is actually more insidious, because it takes advantage of the fact that employees don't actually know what the "average" bonus is, so by tweaking it a few percent is unlikely to be noticed by any individual employee.
Even when the bonus formula is well-known and fairly standardized, to the degree that is consistently paid out over a long period, people start thinking of it as effectively part of their salary. If it goes away or is significantly reduced because of company performance or whatever, it's taking away money people were (even if somewhat irrationally) putting in the sure thing bucket.
