The write protect switch is only a piece of plastic. There is no electrical switch attached to it inside the SD Card to provide a signal to the controller for enabled or disabled.This SD write protect switch only serves a function if the host system checks the position. The original intent was for consumer electronic devices to operate similar to a cassette recorder by verifying which position the SD switch is set and either writing or not writing to the card.
This is fine for cassette tapes, but in practice, people moved the SD card from consumer electronic device which may have honored the write protect to a PC card reader, notebook or other device which was oblivious to the switch.
What a consumer thought was write protected would unknowingly be overwritten by this other system making the switch worthless. This is the main reason the SD write protect switch is rarely, if ever, used.
In fact, on later SD Association products such as miniSD and microSD, they do not have this switch defined.