MSP430 RAM protection
Can anyone explain to me what a strange protection if the password(code) 5Ah is known?


It protects you from random modification by runaway code, e.g. on stack overflow.


  Why RCKEY always read as 69h(not 5Ah)?


So you cannot simply read->write it - part of the protection mentioned above. 

Microcontroller code often contains code that read-modify-writes a memory location to set or clear some bit. With the values 69h and 5Ah it is highly unlikely that such code path triggers a sucessful write in a runaway case.