People are imperfect humans. Someone is gonna forget and check their email, as they habitually do, since we are not trained at birth to segregate our computer usage by machine, while they are at the office (where they spend the majority of the day) on their work machine (where they spend the majority of their Time). Take this example and extend it ....

That said I agree with you and do think being cautious about what one does on ones work machine is very important.