It did not matter how many people replied here, as their decission was already settled when starting this thread.
They just forgot to tell me.
I think i did a basic mistake by posting the join request into their public forums.
Since then, it is advised to ask challenge site admins in private message or email.
I can not really agree myself with their reason for not joining wechall, as i think there is more than a "mild competition" on project-euler. For example they got a ranking especially for the latets N problems.
Maybe they will change their mind one day, but that`s doubtful.
Lesson learned: do not "spam" public forums.
I also thought about a "wrapper" which would enable sites that don`t want to join, but that would imply security issues as sometimes we would need your password for that site.
Also it is not nice to rank them if they do not want to participate.
As a final strike we added the "Not Ranked" section.