And, when you boycott a company, you don't buy a product even if it pass all your functional test/criteria for consuming. But you don't buy it because the product is made by some company you happens to agree within your group/community that you are not going to buy from the company.
The difference is that one of my functional test/criteria is precisely that I want to support the company making it, so it couldn't possibly pass such a test. I don't buy things that are made with slave labor or child labor, that doesn't mean I am 'unfairly' discriminating against people that use slave or child labor :)
I think if companies want to engage in this sort of behavior they should put in the job adverts "Googlers need not apply" or something to that end. At least be forthright with your plans to 'no hire' simply because they may be employed with one of these multitude of companies that you made some secret pact with.
Going beyond that and reporting back to their present employers that they may be looking around is somewhat reprehensible and a breach of confidence and etiquette. People may be 'looking around' for a multitude of reasons, eventually if they plan on leaving their current employ they will need to have that conversation with their employer, but to 'poison the well' by immediately calling their employer if their resume crosses your desk just lacks in common decency and indicates you work in an environment I would never want to be part of, so come to think of it maybe you have done us all a favor :)
What's the different?