People who do this are probably just ignorant (under-skilled), not necessarily antisocial.
Doing this obviously risks your own site breaking if the linked source changes anything or goes away. That’s a serious risk which no reasonable web developer would knowingly accept.
I think the author is rather being cruel in their kindness. You don't enable other people's failings out of kindness, as you are damaging their future.
You have no obligation for other sites' users. The maintainers of those sites have that obligation, and if their site breaks due to hotlinking, it is 100% their problem. You aren't hurting their users; they are hurting their users.
You would be doing them and their users a service by teaching them a lesson. It's not just "anti-social", it's plain dangerous to integrate untrusted/uncontrolled code into your site.
Doing this obviously risks your own site breaking if the linked source changes anything or goes away. That’s a serious risk which no reasonable web developer would knowingly accept.