Anything that shifts the responsibility away from the end user, who is more in control of his own behavior than the ISP is, will reduce even more the incentive for that user to clean up his act.
And making the ISP liable will have a completely perverse cost-shifting effect: users who know what they are doing and take steps to run a tight ship will end up paying extra for the ISP to go around cleaning up machines of the morons.
This seems like a really bad idea.
Steve
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Stephen J. Friedl Unix Wizard Microsoft Security MVP Tustin, California USA my web site