zen-cart has prevention from robots you can try to send different headers basically when you change User-Agent it should work
this worked for me copying from my own browser :
curl --user-agent "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:24.0) Gecko/20100101 Firefox/24.0" -I -3 "www.splitsum.com/zcart/index.php?main_page=product_info&cPath=0&products_id=1"