I kinda figured too that the point of them going back (in time) was to stop the purge, but the people who told them that they had to go back, and interestingly enough that it was to save the people on the Island, were Ben, Richard Alpert and Jacob/Christian. None of those people would want to stop the purge since they were the ones that made it happen in the first place, and other than maybe Locke, it doesn't seem like they would really give a shit about the 815 Survivors living, since they want everyone else off the Island.
I guess it really doesn't matter why they went back, just that they did go back and apparently there is nothing that they could have done to avoid it because you can't change the way that things will happen.
The internal logic seems to be that for the Survivor's their time line is always chronologically forward, meaning that three years have passed for them and they are all three years older and three years "in the future", it's just that the external world of the Island is not in a consistent time frame, thus they are in the year 1977, but they are not in the "past" for them, because they never lived there before. "Time" is just a place setting akin to a location and not a shifting state of existence.
If Hurley shot Myles there in the present 1977, then he would die- but it wouldn't mean that he would never be born at the time that he originally was (at this point, he may have already been born, so nothing that happens now would effect that) and he would still be alive in 2004 because that was his "past". got it?
For Ben 1977 IS his past though, so anything that happens to him then already happened to him and will be a part of his present state.
The interesting thing is that for Locke it hasn't been three years, it's been like a month since he left the Island and then came back (because he traveled into the "future") so technically that means that he's three years in the past compared to the others. that's kind of confusing.
