I was required to submit my taxpayer ID number when I set up my paypal account years ago. Paypal, as a financial institution, is required to submit a lot of information to different Federal agencies, not just the IRS. The reporting threshold you mentioned is just one such thing they are required to report. Most others have to do with patterns of behavior, transactions to and from certain countries, companies, and individuals. Certain keywords associated with transactions can trigger reports to law enforcement. The list goes on. Taxpayer ID number, which for most of us is also our SSN, is required for most of these reports. My suggestion? Deal with it and move on.
edit: I think I should add that any arguement concerning reporting requirements should not be with PayPal, but rather with Congress.