Christmas bonuses have nothing to do with Christmas. Most corporate year ends are at, well, the end of the year. Money has to be paid out by then, if it's budgeted for, to keep the books tidy. Nothing to do with Christianity, everything to do with simple accounting procedures.
If people wish to put on rose-coloured glasses and think that the corporation is gifting them out of a rush of Christian spirit
*snort* ... well, the corporate spin doctors are not going to correct the misconception.
I find the tieing of employment rewards, especially monetary ones, to a religion to be incredibly ironic. It's like you're saying, "Hi, I'm a Christian and Jesus died on the cross so that I can get some holiday time and a little extra cash."