Is the slide-lock on an SD card optional?
Citing from Wikipedia (emphasis mine):


  Write-protect notch
  
  The user can designate most full-size SD cards as read-only by use of a sliding tab that covers a notch in the card. (The miniSD and microSD formats do not support a write protection notch.)
  
  [...]
  
  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.
  
  Cards sold with content which must not be altered are permanently marked read-only by having a notch and no sliding tab.


Moreover, the SD card simplified specifications (physical layer) read:


  4.3.6 Write Protect Management
  
  Three write protect methods are supported in the SD Memory Card as follows:
  
  
  Mechanical write protect switch (Host responsibility only)
  Card internal write protect (Card's responsibility)
  Password protection card lock operation.
  
  
  • Mechanical Write Protect Switch
  
  A mechanical sliding tablet on the side of the card (refer to the Part 1 Standard Size SD Card Mechanical Addendum) will be used by the user to indicate that a given card is write protected or not. If the sliding tablet is positioned in such a way that the window is open it means that the card is write protected. If the window is close the card is not write-protected.
  A proper, matched, switch on the socket side will indicate to the host that the card is write-protected or not. It is the responsibility of the host to protect the card. The position of the write protect switch is unknown to the internal circuitry of the card.