Look at the whole picture. Bonuses are used to attract and sometimes have conditions applied. Someone like PAM has to dig down to attract drivers. The bottom line is you'll pay for your own bonus. The same goes with a newby company that has decked out trucks. Huge attraction for some, but the drivers pay for them.
Why $1200? Why not a round amount? Because they figured an amount they can recover by doing other things. It boils down to something like they figured out how to cheat you out of $100 a week, give you $50 back and they look good.
Unless it's a reputable company that requires 2-5 years experience and offers a bonus, they are blowing smoke at you.