Why are forwards stopping their runs on throw-ins?

There is no offside offence when a player receives the ball directly from a throw-in. That means a forward can begin beyond the defensive line and receive the throw directly. Normal offside law applies again as soon as a teammate plays or touches the ball after the throw. At that next moment, the forward's position is judged in the usual way. I would encourage the players to exploit the direct throw-in opportunity, but also to keep moving after the first touch. Standing beyond the line is useful only if the thrower can find them or if the movement creates space for somebody else. The simple message is: no offside from the throw itself, but the exemption ends once the next teammate plays the ball. The exemption applies only to receiving the throw directly. If the forward lays it back and then receives another pass, offside is judged normally when that next pass is played. The player needs to reset or time the next movement rather than assume the exemption continues through the attack.