I think most CPA networks do ask for it because if you make more than $600 they have to report it to the IRS. Like the guy above said, maybe you can find some offshore networks that don't require it, but for the most part if it's a US company you're going to have to give your SSN.
If you want a little more protection you can apply for an LLC (limited liability company) that has it's own SSN called an EIN (employer identification number). That way you can apply to CPA networks without giving your SSN. Every CPA networks accepts EINs in place of SSNs.