It does seem like an odd thing to be worried about. I think it's because the ability for the AI itself to do things is what makes AI different from other future technology. For example, a Star Trek replicator could give its owner just as much power over others as a general-purpose AI, but it's not going to decide to exterminate humans on its own. Nuclear weapons were thought by many to have similar properties back when they were on the horizon.

In other words, the less likely scenario has more novel elements, and therefore more interesting to think about. Therefore, it gets more attention than it normally would given the probability of it happening.