When Alex was killed, Ben was shocked and said "He changed the rules". Maybe ben and others can go into the past, but the rules made by whoever knew about time travel (widmore, ben?) were to never change the past even if they do have the ability to?

I posted something about changing the past before, and someone told me the lost writers made it clear you can't change the past and referenced an episode that shows this, but maybe they "can't" change the past because of rules, but "can't" doesnt mean it's impossible?