ENFJs may stereotypically focus away from themselves for 'the greater good' and all of that jazz, but it's doesn't have to hold true or be completely selfless. I would imagine that either your definition of them is held too close to the typical read-out or you're just running into the wrong sorts of people who just so happen to be ENFJs.
That's my opinion though. I've become a bit picky and abrasive in the way i help others, but i don't expect much in return in the first place. Are you more concerned about the ENFJs because of the guidelines of their personality type? I've found this problem holds true in almost every personality type simply because humans aren't these well-oiled machines beyond the labels we can give them.
Perhaps they appear fake because they are trying too hard to please others and gain their own benefit, as well, without realizing how difficult it is to maintain both at once. As far as i know, the middle-ground is hard for ENFJs to find, but we have the ability to be so outwardly focused to see that in ourselves. It would cause a selfish-seeming issue.