People colluding to gain 51% is not an attack. It is a hostile takeover. This is done in businesses where a company buys 51% of the shares of a organization. What that company does after it has control is up to the owners.
51pool.org is doing just this. It is providing the opportunity for people to collude together to "purchase" a business. Once they have 51%, they own the network and are absolutely allowed to control their network how they deem fit. Right now, ETC is 100% colluded with people operating the network how they decide. Someone coming into to take 51% ownership is no different than the collusion that is happening right now on ETC or ETH or BTC