There have been proposals to track the location of trucks at all times, presumably so that a truck that drives all over the country is paying road fees in the states where it's actually driving instead of to whatever other state it's registered in (which would presumably be the one with the lowest fees). Levying the fees at the charging point replicates the existing (imperfect but pretty good) model with fuel tax because you at least have to charge somewhere that you're actually driving that day, without the privacy implications of tracking the vehicle at all times.