It's like email. If you press unsubscribe you just confirm the email is valid. If you send STOP back you just confirm there's a human reading the messages at that number.

Besides, this seems to be an US only thing so it will only work for law abiding US based spammers^H^H^Hdirect marketers. Not for spammers outside the US, US based spammers that don't care about the law or scam/phishing messages.