I did not say that it is equal to a 51% attack in all ways. Whilst what you say is true, a 51% fork is also an attack. Anything which causes a blockchain split, especially one that is this severe, should be seen and treated as an attack.
I really don't see why a hard fork is an attack. It is a divergence in the protocols. With a hard fork, untenable tensions of disagreement, which don't allow a consensus of second order (that is, a *protocol consensus*, not a block chain consensus within a given protocol), are relieved. The market cap splits according to how "people vote with their money". No-one actually loses money, because you are double coin holder at the fork. And now, two coins exist, with different protocols, each according to their communities.
There's nothing wrong with hard forking, and the "drama" of ethereum has shown us how well this turned out to be. There was an unreconcilable tension in ethereum: should one stay with the original axioms of "code is law" and "unstoppable contract" which were the value proposition of ethereum, or should one, in "unforseen circumstances, when many rich people were going to lose money, accommodate the rules" ? The split was very healthy. We have the original, unstoppable, code-is-law ethereum (ETC), and we have the "we accommodate the big investors if something totally unexpected happens because this was just a beta-test" ethereum ETH. Visibly, the market (made by the same rich people) voted for being flexible enough to get one's DAO funds back, and a small minority stuck with unstoppable contracts where code is law.
I think the bitcoin tension is sufficiently high to have a hard fork too, and have an original bitcoin with 1 MB blocks, and another altcoin-bitcoin that accommodates more transactions. But bitcoin deriving most of its value from "being the first" and no other value proposition that is not filled in by an altcoin, this will probably not happen, because the "forkers" would totally lose their "first mover" value proposition, which is the sole thing bitcoin has going for it (but is important), and just be holders of just another crypto alt coin.
So, until bitcoin has lost its first mover value, this will not happen.