How can this prevent bots? Are bots not able to do the POW? Just curious..

Same question. Bots denied because of this will just be re-developed to be able to perform the necessary work and continue. It's a hurdle today for the bots because they haven't gone up against it yet, but once the bot developers keep seeing these, they just add in whatever else it needed to overcome the hurdle. That's why there are more complicated captchas (i.e., human intelligence tests) now versus the simpler ones from years ago that were easily defeated after putting some thought into it. i.e., it has always just been an arms race. This is nothing new that's going to end the war between site owners and bot developers.