Yeah, as you suggest by your "slightly safer" parenthetical, the mechanical write protection on most SD cards is just a suggestion that the card reader is free to ignore (though obviously most don't).
It's actually specified in the SD spec [1], which says on p. 38 that the mechanical protect switch is "Host responsibility only" and that the "The position of the write protect switch is unknown to the internal circuitry of the card."
The popular CHDK custom firmware for Canon compact cameras subverts the write-protect tab on the SD card to be a CHDK on/off switch. So it's widely known that software may ignore this switch.
It's actually specified in the SD spec [1], which says on p. 38 that the mechanical protect switch is "Host responsibility only" and that the "The position of the write protect switch is unknown to the internal circuitry of the card."
1. https://www.sdcard.org/downloads/pls/simplified_specs/part1_... [pdf file]