I don't think they have any reason for that, SSN is just a # used by Social Security Administration supposedly for internal purporses, in early days they even had 'not to be used for identification' written on cards. You can even send them a form (I think it's called 'Withdrawal of Benefits') and opt out, so it's not even required. Why would a private company refuse to pay you simply because you are not a participant in optional government program? Since not all affiliate programs require SSN, then obviously it's not a legal requirement. I don't have an account, but if I did I'd ask their legal dept. (or maybe have some fun and send them an invoice by certified mail return receipt requested )
I think if you form a nevada corporation you can get an EIN from the IRS even if you are not in or from the US but better check up on that. Theres some tax treaties to consider too, its a complex business these taxes and tax ID numbers.
EIN = FEIN = Federal Employer Identification Number. Its supposed to be available in minutes online from the IRS for US people. In my opinion its valuable even if you dont have a company because it prevents any tom dick and harry that you deal with from getting that one sensitive number of yours in their hands---they could lose it to someone unscrupulous like a hacker or a shady employee... It just disturbs me to know that of all the small companies that you do business with...many of them want your tax ID number and the more of them that demand it and GET it...the more your chances increase of getting your identity and credit hijacked.
Hey all I know is my google account would NOT pay me until I punched in my SS#. And its not just google...every tom dick and harry wants your "social" before they will be "nice" to you. Its actually illegal for them to demand it but just last night I called comcast tech support and the guy was whining because I wouldnt give him the last 4 digits of my SS# for "verification purposes". I told him hell, I never gave comcast that number in the first place so you sure as hell can't verify anything if I never gave it to you in the first place and he sat stunned...and then asked me to give him my account number from my bill. Its ridiculous...EVERY little tiny business just wants to get their hands on your SS# because it makes them feel important or something. And what annoys me is if anyone has your SS# they can wreak havoc a lot easier which is why I hate handing it out to these companies in the first place.