I doubt Chinese students are any threat to US security (they leave that to the professionals...) but the Chinese goverment sees them as a threat to itself.
It is so nervous about its students studying in countries like the USA and UK that it keeps them under surveillance.
Any infraction in its eyes (such as being seen anywhere near a protest against the CCP) is likely to result in punishment by action against the student's family, such as withdrawal of employment or pensions. It might also recall the student immediately.
Though might the Chinese intelligence services also use them as an unwitting way to identify host citizens who might be useful to know of, like friends' parents working in sensitive areas? The politicians presumably think they have identified particular hazards from the Chinese presence.
