My ISP filters malicious emails. However, those get stored on their server before they are forwarded to me. That gives the ISP computer a chance to run virus checkers against them as a static document.

If they were virus checking real-time data that is just flowing through their routers, I really think that would slow down browser page loads a lot. Even worse, gaming would slow down. I'm not a gamer, but know enough that they would find the extra delay to be unacceptable. I have no idea how movie streaming would be affected. Can they virus check as it's going by, or do they have to buffer the entire movie to check it first?

Maybe the ISP could whitelist certain sites, but ask our friendly monitors about how hard life gets when a 3rd party decides they are malicious and won't let traffic through their site (Hotmail emails won't let email from NAT through to their users).

Bottom line, I don't think the tradeoffs would be acceptable to me.