There is an informaton tab that explains that Google Shopping is becomming a Commercial site.
This means that they can permit or restrict anything they want on it similar to eBay's rather restrictive stance that they held until they realized the amount of revenue that they were missing and then they relaxed their policy slightly.
While the main Google site returns all the hits you can imagine on just about anything, Google shopping does not. I would imagine that Google Shopping generates revenue from merchants that sell their products via this portal. If Google chooses to exclude firearms related merchants and products, that's their choice.
I would choose to avoid sellers that use the Google Shopping as an outlet and inform them that I will not be purchasing their goods or serivces because of their use of Google, explainign why the Google Shopping restrictions are unacceptable to me. You also have to be willing to NOT spend your money with that merchant, otherwise it's not effective.
You have to remember that Google and the merchants using Google are after the dollar bills in YOUR wallet and that YOU hold all the cards in the game. If you advise them that you will be spending your dollars elsewhere and WHY, you may make a difference. Multiply that by a few hundred thousand or million people, you have the beginning of a force that can bring about change.
So rather than punish Google for being foolish, advise the people that PAY Google to use their service that you will not be making your purchase from them because they use Google. If you're that business and you get enough messages from people that are not buying from you because of Google, your going to pay attention and maybe make changes.
Don