The reason for those is to then quickly provide a way to invalidate their visa and have an administratively easy path to kick them out without having to go to court and such. Terrorist activities doesn't mean blowing things up, it could be open to all kinds of interpretation (donated money to some organization, protested and was arrested for rioting, went to a black-hat conference which had talks about subverting security systems of power plants etc, etc).
Any statue establishing those could also establish that those are proper cause for immediate deportation, no need for fancy forms and logic-gymnastics.
