can't I write the first 16 bytes because this is where the scack pointer and registers are?

Sure you can; however since they are other things as well, before you read them they may have changed before you read them.

an example:
mov r0,#7
mov a,#11
MOV @R0, A ; inst 1
mov r7,#44 ;inst 2
mov a,@r0 ;inst 3

inst1 writes 11 to memory location 7
inst2 writes 44 to memory location 7
so inst3 will read 44 to tha acc, not the 11 you expect

Erik