Some quests are supposed to be solo - just you and the boss. BUT While your friends can't join, you CAN buy henchmen.
Why is it okay to have 3 level 60 henchmen in a solo fight but not okay to have 3 players with you?
Some towers are no port. BUT If you are already in the tower, you can go back in the door and run back to where you were.
Why is it okay to walk back in but not port back in?
I might be wrong, but I think you can port back in with the "mark location" button. However, the traditional port to friends is disabled, because I think the computer cannot distinguish between friends (those that made to the same step as you and those that are porting in for the first time). I believe the "quest check" is accomplished at the sigil for the dungeons that have friend porting disabled. My guess anyways.
And for your henchmen comment.... since henchmen usually mess up the fight more than assist, maybe they allow it for replayability (if you know what I mean).
