Well, it looks like they've admitted an error between US and EU rulings this year, and it's only fair they stick by it for the season. If they let you play for your title, then a player who qualified early doors could argue they should have been allowed to play in later events, maybe in their home nation, to go for their own national pride. It's clear you want your title though, and it's a shame you won't be able to fight for it.
In a way though, I can sort of see why they do it. Players you knock out in top8, or earlier, could feel pretty hard done by, knowing you had to pass up the flight late in tournament so as not to take a prize and such. It seems that once you're into Worlds for the season, they don't want you taking more prizes, making things a bit.. fairer (Not sure if that's the right word) I guess for other players.
Either way, it's not like you can't go and support your friends at the weekend, so just go, and have fun. At least the pressure is off for you!