It only applies to programmatic and automated messages

When a company or service sends text messages using an automated system like SMS marketing software or bulk messaging service), these platforms are by design and mandatorily programmed to recognize keywords like "STOP" as a signal to unsubscribe the recipient from future messages. This is a requirement for all such systems under Telephone Consumer Protection Act (TCPA).

Text messages to your friends do not go through such platforms and so are not subject to that law.