let's ask allmighty wikipedia:
[the nx-bit] is used to prevent certain types of malicious software from taking over computers by inserting their code into another program's data storage area and running their own code from within this section; this is known as a buffer overflow attack.
That's why DATA memory should NOT be allowed to be executed. You say there is no way to let users input new commands? But in your next sentence you tell us how... well, the segmentation fault should occur if you try to execute data memory.
