Nothing prevents playoffs, that is just the system they use to determine champions. I am more discussing the bottom side. You can have playoffs at the top and not impact the relegation at the bottom. The Mexican soccer league has both playoffs at the top and relegation at the bottom, I think Brazil does too but I don't follow the south american leagues. There are also often playoffs to determine promotion in the lower divisions.
this is true, which is why my favorite leagues in fifa 08 for ps2 to play in are the mexican and usa mls.