Under normal times I agree Universal or Disney buying Six Flags or Cedar Fair would not be allowed. Seaworld would be because it doesn't have that many parks. However, because of the economy today the Federal Government and States would approve anything that would save jobs.Cedar Fair is in better shape than Merlin as it stands, in fact last I heard, Cedar Fair was looking to acquire either Six Flags or SeaWorld parks. The funny part about that is Six Flags actually made an offer recently for Cedar Fair.
But the bottom line Disney or Universal most likely wouldn't be able to get it past the regulators to buy any of their competitors, as it would create a monopoly.