I think because they are an apex predator they wreak havoc. Once they get to a certain size they have no natural predators.
As a side note, they are significantly stronger than any animal in their weight class. They are 1/10 the weight of a 2000 pound Bull and yet they possess comparable strength. I am not claiming that wild Burmese are a major threat to humans, however no one wants a foreign animal with that type of potential wandering around outside. When someone buys a cute baby Oscar for their fish tank and it grows too big they either keep it, try and give it to a pet store (who are generally not interested in adopting large fish as this is an ongoing issue), or worse case scenario they toss it. When this happens with a very large constrictor it is not quite so simple.
I don't want any more ridiculous laws to worry about just as much as the next guy. By the same token I don't think someone without legitimate experience should be in possession of an animal this size. To hope/assume that someone purchasing this type of animal will exercise the proper responsibility is not enough in my opinion.
Its illegal to own a wolf and yet we own dogs. It is also illegal to own large cats, yet we own small domesticated ones.
Alternatively, we are pretty much able to own any constrictor, including anacondas and retics. I don't see how people complain about biased laws in this case when it is clear that the bias is currently in favor of our beloved reptiles.