Free divers actually avoid the decompression problem altogether. They breathe in air at surface pressure, it compresses as they dive down, and uncompresses to the same size as they rise up. The air cannot bubble out of their blood.
The problems arise when you breathe in high pressure air down deep and bring that air up with you.
While it is highly unlikely that a single free-dive would cause a symptomatic decompression injury, there is a high incidence of decompression-related injuries amongst free-diving fisherman due to repetitive and deep free-diving. The probability of decompression sickness occurring is related to time and depth.
It's a matter of time and volume - it takes time for nitrogen to dissolve and they are not down there long. And they only have a single lungful of air, so there is not a lot of volume to dissolve.
But, if they do repeated dives, each one will dissolve more and more nitrogen until they do have problems.
The problems arise when you breathe in high pressure air down deep and bring that air up with you.