It is bad, yes, but not as bad as people think. A four foot long T-8 fluorescent bulb has 3-5 mg of mercury. About 95% of that mercury is in the form of a solid phosphorus powder. So, you get about 0.2 mg of mercury in the air and about 3.8 mg of mercury in solid form per bulb that lies harmlessly on the floor.
As a comparison, 1 kg of trout has about 0.3 mg of mercury (in solid form). 1 kg of shark or swordfish has about 1 mg of mercury. The amount in a fluorescent bulb is not much more than the amount in foods that people purposely ingest.
True, vaporized mercury is worse than solid mercury. And true, they are breaking multiple bulbs. But, still with adequate ventillation, they aren't THAT much worse off than what they are just from their normal diets. The problem comes if there are many battles with many bulbs in a small, closed room. Since I don't know the sport, I can't really say how it is done.