It's not uncommon for a fire crew to advance into an exposed position to make an attack or search for victims. As was the case here, there is usually a hoseline or two set up in a defensive position, covering their escape path. The difference is, this is usually done in a comparatively small space (i.e. a crew holding the staircase of a house while a second crew searches the upstairs). If the defensive position starts to look at risk, the exposed crew can make it back within a minute or two.
The crew in the northern tower of Notre-Dame would have taken far longer than that to make it back through, and it's very likely that if the defensive position had been lost, it wouldn't have been recognized until it was too late, trapping the crew in the tower.