Every place I've received a bonus from gave me a gigantic, in-very-clear-terms, no-nonsense page to sign that said I acknowledge the this bonus is discretionary, it is not to be expected, there is no expectation to ever receive another one, etc.
This is most likely done because everyone expects to get some kind of a bonus (if you don't receive a bonus, unless the company is basically going bankrupt, that's a really, REALLY strong sign that you should probably your bags up and GTFO). It is considered part of the basic pay package, but then again, one of these years, there might not be a bonus, and they want to make it absolutely clear that you can't expect one in that case.
The reason for this is that most hedge funds pay bonuses out of their performance fees, and if you have a shit year in the market, there isn't any money to pay bonuses.
If a company does this in Europe [I think generally in the EU], I believe it would then be considered a contractual entitlement and part of your salary - so the company would legally have to pay it anyway.