-=- This post is in reply to a comment on the Huffington Post article linked at the bottom of this post-=-
There are multiple reasons that ISPs don't run antispyware/antimalware before traffic reaches customers, many of them probably legal. Like Spifflous mentions, they can do DPI (Deep Packet Inspection
ISPs could also run a black list on these sites. It takes a lot of work to keep up with the criminals and keep a constantly changing black list. That said, there are companies that do it. I use OpenDNS which uses crowd sourcing for it's blacklist. I like having control over it. The next question would be who controls the blacklist, and what do you do if your site is wrongfully blocked. How about if your site is correctly blocked because of malware due to being hacked, but you fix it. Who determines what needs to be done to have it unblocked. At what point does blocking sites become censorship
If you want a technical reason, DPI slows down internet traffic. Technology is getting better and there are devices that can do DPI in real time, but the cost more. If ISPs did do this, who do you think would end up paying for it?
Read the Article at HuffingtonPost