It doesn't really matter much *why* they want it. If you want to shop for them you need one. There's nothing nefarious about it. Go to the New Mystery Shoppers section and read the thread on taxes for more information on the subject.
You are using your SSN because you were grandfathered in. For a couple of years new shoppers have no other option than an EIN. As of now Trendsource is the only company requiring one.
Equal rights for others does not mean fewer rights for you. It's not pie.
"I prefer someone who burns the flag and then wraps themselves up in the Constitution over someone who burns the Constitution and then wraps themselves up in the flag." -Molly Ivins
Never try to teach a pig to sing. It's a waste of your time and it really annoys the pig.
People who file a schedule C *are* in business and can get an EIN. It stands in as a redirect to their SSN for tax purposes. You don't have to have one to file a schedule C, but you can get one and it doesn't cost anything. Trendsource happens to require it, maybe because they see people with an EIN as being more committed to their business than people who don't. We don't really know why they want it. But if you want to work for them, you need it.