The array is processed and all names for the id's are done and an input box is made for each one and if it is selected it is checked. Just the way it is done. Yes that is the only thing that allows for multiple selection as select boxes and input boxes are one selection only.
Now if you want only the ones to be checked to show do this in showproduct.php
Code:
Content visible to verified customers only.
change to this
Code:
Content visible to verified customers only.