There's a difference between something that's physically impossible and something that's technologically impossible.
If something is physically impossible, there's no need to prohibit it, because no one will ever violate it. Although if there's likely danger from trying it, you might want to prohibit attempts, to protect the subjects of the experiments from that risk.
But if something is technologically impossible at the current time, you may be able to envision scientists and engineers overcoming those limits in time. If you believe that you won't want to allow it when it becomes possible, it makes sense to pass the law against it now, rather than waiting until after the fact. This might be especially true if you're concerned that the party in power when it becomes possible would have a different view, and wouldn't prohibit it then.
It might also be easier to pass the law now. Since no one has skin in the game yet, there's likely to be weaker opposition to it. There won't be any lobbying groups, for instance.