One justification might be that it creates an environment where everyone but the ad provider loses.

If, hypothetically, Google was more pervasive than it already is, and made their ads more intrusive until consumers overwhelmongly clicked the highest priced ad for a given keyword, and Exotel bought the Ozonetel keyword, Ozonetel would have no choice but to buy ads for its own name. They could also buy ads for the Exotel keyword. The two could get stuck in a bidding war, wastefully sending money to Google when the optimal thing would be to just send users to the site they're trying to reach.

We're long past the point where you should expect users to try to guess domain names and to pretend that Google is just another general-purpose search engine. That universe could be nice but we don't live in it.