The short answer is this:
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If the only people who signed up with rakeback affiliates were those who were signing up for poker for the first time, Party would not have a problem.
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In fact, the vast majority of people who sign up with rakeback affiliates ALREADY have poker accounts in their names. This means two things:

1. They are violating the terms of service by opening multiple accounts, generally with false information on at least one of them. Good luck finding scammers or credit card fraud when half your players have false info in their accounts. You gonna delete half your user base?
2. The person who marketed and generated the FIRST account is getting screwed out of the revenue. This may be Party itself, or may be some affiliate who advertised and produced content. Either way, someone spent time and money to catch that first sign-up, and now they're losing it. They're not going to be happy.