I agree that they should still be considered a debt collector - to the person who allegedly owes a debt, it's the same  - but apparently the law drew a distinction between "creditor" and "debt collector".

I didn't research very deeply, but I found this:

http://www.marketwatch.com/story/supreme-court-wont-widen-debt-collection-abuse-law-in-santander-case-2017-06-12

"The case hinged in large part on the definition of “creditor” and “debt collector” and whether a company that buys debt should be treated as a creditor, not subject to the collections law. The law applies only to companies that collect debts on behalf of others—an $11.4 billion industry—and doesn’t apply to businesses like Santander who buy the distressed debt from other companies after it defaults, the Supreme Court ruled.

The plaintiffs argued that unscrupulous debt collectors could evade the law by buying the debt."

I don't like it either, but given the unanimous decision I'm guessing the law gave them little choice.