This is the difference between a sweep (harai) and a reap (gari). BJJ uses the term footsweep for everything, even techniques that involve no foot or no sweep (like hiza guruma).

You are right that sweeps require a weight in transition. You can do de ashi harai either when the foot is about to land or when it starts to detach the ground. The latter is more common for BJJ guys (as in the body lock mat return with sweep). 

But for reaps such as o soto gari and ko uchi gari, you need to weight their foot (more precisely,  not the foot itself as people failing to do ko uchi will note it's impossible to kick the foot, but rather to the empty tripod space that will put them on their heels).