It is. I am a system administrator and more than once I had problems with one of my systems - which is restricted use and very important - caused by rogue javascript code inserted by ISPs or viruses on the user's computer (such codes could never harm the system server itself but prevented the affected user to use it). And as example, if it was you who put this rogue code on the user's connection I could have you arrested for interfering with state emergency services. Point is, injected rogue javascript can cause many, serious problems.