There is one reason for this:
They don't want extra competition for the ad search terms, since they use them themselves. That drives up the price per click.
But this time I think they did you a favor anyway, since free options would be a lot more likely to actually result in a profit for you.
Remember that a new customer is much more valuable to Shutterstock than for any single contributor so they can spend more money on advertising. They get their money for life, you only get a one time referral percentage.