I'd say the vulnerability of all other mined coins to a 51% attack (or some other type of attack) by Bitcoin miners makes this impossible.
Are you saying that launching a 51% attack on a currency is a perfectly legitimate tactic for advocates of a competing currency to employ?
I would say that it does not matter whether it is 'legitimate'. The salient point is that it happens.
It's not likely to happen on an organized, large scale through pools and the like if the community considers it an illegitimate tactic. Hence my question. Some might argue that it's theft, fraud, criminal abuse, or the like.
I would say it's in no way illegal, at least in its primitive form. You can't accuse me of mining with more than 50% of the network's hashpower, and you can't accuse me of refusing to process other's transactions because I don't want to use my own hardware to do that.
