Most SD card "write prevent" tabs are suggestions only. From Wikipedia:
The presence of a notch, and the presence and position of
a tab, have no effect on the SD card's operation. A host
device that supports write protection should refuse to
write to an SD card that is designated read-only in this
way. Some host devices do not support write protection,
which is an optional feature of the SD specification.
Drivers and devices that do obey a read-only indication
may give the user a way to override it.
In most cases the switch is just software detectable. Some prosumer cameras use this switch to indicate there is a firmware update on the card and that the camera should try to apply it when it is powered up.
