They don't have to issue overt threats for this to be intimidation. It would be similar to them torching his car or leaving some other well-understood method of intimidation. Burning crosses come to mind.
In any case, whoever did this shown that they know where this person lives, and they're willing to break some laws / do property damage in order to silence him. By going after his "internet cable", they are clearly referencing his internet postings / blogging activity. Sure, this doe have the effect of censoring him (at least until the cable provider can fix it), but they're also sending a warning that next time, they might do something more severe.
What's extra nice is that by them not leaving a note, he has nothing to take to the police.