How so? If you make money through Google Shopping you should follow the rules. If you don't, well then no loss to you.
In business a photo can often make the difference between a sale or no sale, or more important, which shop the sale goes to. Some businesses, like fashion, are almost entirely "photo" based. Others, like food, cars, etc. rely on it extensively.
Here's an example in which, for arguments sake, we state that all non-photo factors are equal and the best they can be.
Your business spends $100,000 a year to have top quality photos taken for the products you sell and you find the photos lead to a five to ten fold improvement in sales vs. stock or no photos. Now say all your competitors just steal your images and use them for their sites. Your sales drop back to where they were and your advantage is gone. And now you've got to make up the $100,000 to top it all off.
Having a watermark won't stop lesser competitors from sealing your image, but other "on par" competitors will likely not due to potential litigation. If you don't have a watermark everyone steals it with the line, "I didn't know it was something I couldn't use ... my web guys just found it on the internet. I'll talk with them." (or whatever) So now you have to constantly police the internet for competitors using your images, deal with take-downs, and lawyers.
Hope I've cleared things up a bit.
It's a pretty easy equation--do you make enough money through Google Shopping that it makes up for any potential losses from others stealing your image (which you can of course still go after legally)?
Not happy with NSA collecting dragnet surveillance on you? It's a pretty easy equation. Renounce your citizenship.
Neighbor making too much noise at night? It's a pretty easy equation. Move out (or kill them?)
Why can't someone that's party to a relationship make a complaint about an aspect of that relationship?
In other words, a cry for Google to change the policy to minimize the problem rather than just abandoning Google Shopping.
The answer isn't to stop using Google Shopping, and the answer isn't to 'put up or shut up'. I would hope that someone at Google Shopping might be able to figure out a resolution with OP that is the best for everyone involved - i.e. doesn't break google's TOS, and doesn't leave OP paying to produce high-quality photos that his competition will steal.