The power of arrest has and is abused to punish and harass people. Perhaps I'm mistaken and in the UK those accused of unauthorized downloading are subject to arbitrary arrest, but this all seemed very abnormal. The police only investigate a fraction of the crimes reported to them, even when there's evidence or they they think they could get a conviction.

If you called up the police, any police anywhere, right now and said "Officer arrest this man, he downloaded a pdf from my server and I have the log.txt file to prove it!", I would honestly be willing to wager the deed to my house that the police would not be banging my door at sunrise to forcibly take me away and interrogate me.

At best it was gross incompetence, at worst abuse of power.