my 99 montero/sport

yes, i just bought my montero in september and i was told that i had to bring it to the dealership to program in the code for the stereo well i did and after i left i went to town and when i got back in it was gone again so i went back to the dealership and they reprogramed in my code and told me the code incase it happened again.. well now everytime i get out of my car for a long period i have to recode it and program the channels all over again and i was wanting to get a new stereo with a system and i was wondering why i have to keep programming it in and if i took the stereo out will it mess up my system they also told me if my battery is disconnected for more than 15 seconds i would have to be towed in the dealership to reprgram my whole car cause it would shut off and i would not be able to re start it is this try and why if so?:uhoh: