They have done this in the past to many others; look up the Sprint - Cogent fight a few years ago.

Cogent want peering (i.e. free traffic) with as many providers as possible, but it usually doesn't work out for their peering partners because Cogent's traffic is unbalanced (up / down aren't roughly the same).

Usually when you don't have a direct peering arrangement, you route there through other providers, but Cogent specifically blackhole other networks to try and force them to peer.