You mean the developer atm: infistar.
He specifies in his license terms:
And he gets away with selling this by saying you are paying for his time not the script. That is because it is against Bohemia's EULA to sell scripts, mods or addons for profit. However, the EULA does state that script, mod and addon creators are granted ownership of their work. That means he is allowed to prevent people from editing it or posting it if he wants to.
His antihack is really just hangender's and monky's original work that he edited, so it is kind of silly he doesn't allow editing by admins. Admins need to edit it to make fixes specific to the mods their servers are running. They also probably want to customize many aspects of it such as admin menus, admin scripts, logo names, addaction and display whitelists, which files to scan for, etc.
The point you made about less people having the source code is not really valid. There is nothing stopping cheaters and hack developers from buying it and then sharing it with all their friends privately. If it is posted on a public forum it will obviously be taken down, but that still doesn't stop all the private sharing that will inevitably occur.
And it is worse to not allow admins to edit the script. Admins can change the variable names for all the most commonly exploited publicvariables. They can limit the display, addaction and commandingmenu whitelists to only ones that their server uses. They can change the names of common ones used in mods like selfbloodbag, deploy bike, freds wardrobe, etc. for better security. If you leave them on defaults smart cheaters know the exact variable names to use to give themselves super admin, disable the antihack, kick/ban, use menus, etc. And this has been done many times in the past by hack developers. Hiding the source code is not a real solution.