Yeah, I've caught managers doing the same thing. I may have opportunity to call them on it coming up.
So the employer can refuse to pay the bonus, but they have to give a valid reason. There are two levels to this: first, basic employment law, which places very hard limits on what you can do. Second, many companies (like this one) have agreements between the works council and the company, agreements that regulate these and other matters and become part of the employment contract, and are therefore also legally binding and enforceable.
In all cases, the employer has to have valid, provable performance-related reasons, these have to be communicated to the employee in a timely fashion and the employee given a chance to improve their performance.
In my case, management was so clueless that they knew none of this, they thought that bonuses were gifts they can shower on people they favour on a whim, just like a king to his subjects.
It was a bit of a rude awakening. :-)
They then tried to create a "case" retroactively, but apart from the fact that there was no case, it also wouldn't have mattered at that time.
You can cheat around paying more than the minimum bonus by picking yearly targets (quantifiable like for sales) which are absurdly high.
I never had a bonus, so please take this as 3rd hand information.
"You didn't do this assignment"
"Er, yes I did, here it is"
"Well, you didn't keep me up-to-date on it"
"Er, here are my weekly status report e-mails"
"But you know I don't read my e-mails"
Huh?
Another good one was the following: I had assigned one of my reports a task before going on vacation. During said vacation, my "manager" pulls said report from that task. Then afterwards dings me for that task not being completed.
Huh?
"We're not paying you your bonus"
"Why?"
"You have to know that yourself".
"I do not, and no, that's not how this works"
"I won't tell you"
Fortunately that was in Germany, so not just crazy, but also illegal.