Because they can and will live in American waters without too much adaptation required.
And most importantly because they can and will kill everything in these waters.
The US seem to want to protect their native species, can we blame them?
So yep, they're banned from import, export, you can't own them, nor carry them alive in a truck or anything else. You are allowed to fish for them, if and only if you kill them right as they come out of the water. As far as I remember, you are not even allowed to put them back in after you reeled one out.
I believe Channa Argus is the reason this ban took place, because it could live as far north as Canada. But you can find Micropeltes and other tropical channas in some states further south.