This is working as intended. Due to the way it is designed Guild Wars is more competitive. Guild Wars is also inherently different in the back-end and it structured very differently to the other live events. Technically, it becomes complicated to feasibly allow someone to join a Guild War mid-week vs a live event. Additionally, the other events are focused on Guild participation and cooperation so the natural inclination is that a player can join and contribute where possible. I would think the exploitation risk is also contrasted by the fact that leaving a Guild at all comes at cost of Trophies, Seals and other contributions to the entire Guild.