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For the same reason most fire trucks in the USA have plastered on the back "Remain at least 300 feet behind vehicle".

... "Emergency vehicles" such as fire trucks and police vehicles respond to Emergencies. Emergencies are by definition unsafe.

So you could "follow" (from very far behind) -- but it's implied often times that individuals are following closer than permitted in order to provoke a response, or simply without discretion to the publics safety.